web3-proxy/web3_proxy/src/rpcs/connections.rs

///! Load balanced communication with a group of web3 providers
use super::blockchain::{ArcBlock, BlockHashesCache};
use super::connection::Web3Connection;
use super::request::{
    OpenRequestHandle, OpenRequestHandleMetrics, OpenRequestResult, RequestErrorHandler,
};
use super::synced_connections::SyncedConnections;
use crate::app::{flatten_handle, AnyhowJoinHandle};
use crate::config::{BlockAndRpc, TxHashAndRpc, Web3ConnectionConfig};
use crate::frontend::authorization::{Authorization, RequestMetadata};
use crate::jsonrpc::{JsonRpcForwardedResponse, JsonRpcRequest};
use crate::rpcs::transactions::TxStatus;
use arc_swap::ArcSwap;
use counter::Counter;
use derive_more::From;
use ethers::prelude::{ProviderError, TxHash, H256, U64};
use futures::future::{join_all, try_join_all};
use futures::stream::FuturesUnordered;
use futures::StreamExt;
use hashbrown::HashMap;
use log::{error, info, trace, warn, Level};
use migration::sea_orm::DatabaseConnection;
use moka::future::{Cache, ConcurrentCacheExt};
use petgraph::graphmap::DiGraphMap;
use serde::ser::{SerializeStruct, Serializer};
use serde::Serialize;
use serde_json::json;
use serde_json::value::RawValue;
use std::fmt;
use std::sync::atomic::Ordering;
use std::sync::Arc;
use thread_fast_rng::rand::seq::SliceRandom;
use tokio::sync::RwLock as AsyncRwLock;
use tokio::sync::{broadcast, watch};
use tokio::task;
use tokio::time::{interval, sleep, sleep_until, Duration, Instant, MissedTickBehavior};

/// A collection of web3 connections. Sends requests either the current best server or all servers.
#[derive(From)]
pub struct Web3Connections {
    pub(crate) conns: HashMap<String, Arc<Web3Connection>>,
    /// any requests will be forwarded to one (or more) of these connections
    pub(super) synced_connections: ArcSwap<SyncedConnections>,
    pub(super) pending_transactions:
        Cache<TxHash, TxStatus, hashbrown::hash_map::DefaultHashBuilder>,
    /// TODO: this map is going to grow forever unless we do some sort of pruning. maybe store pruned in redis?
    /// all blocks, including orphans
    pub(super) block_hashes: BlockHashesCache,
    /// blocks on the heaviest chain
    pub(super) block_numbers: Cache<U64, H256, hashbrown::hash_map::DefaultHashBuilder>,
    /// TODO: this map is going to grow forever unless we do some sort of pruning. maybe store pruned in redis?
    /// TODO: what should we use for edges?
    pub(super) blockchain_graphmap: AsyncRwLock<DiGraphMap<H256, u32>>,
    pub(super) min_head_rpcs: usize,
    pub(super) min_sum_soft_limit: u32,
}

impl Web3Connections {
    /// Spawn durable connections to multiple Web3 providers.
    #[allow(clippy::too_many_arguments)]
    pub async fn spawn(
        chain_id: u64,
        db_conn: Option<DatabaseConnection>,
        server_configs: HashMap<String, Web3ConnectionConfig>,
        http_client: Option<reqwest::Client>,
        redis_pool: Option<redis_rate_limiter::RedisPool>,
        block_map: BlockHashesCache,
        head_block_sender: Option<watch::Sender<ArcBlock>>,
        min_sum_soft_limit: u32,
        min_head_rpcs: usize,
        pending_tx_sender: Option<broadcast::Sender<TxStatus>>,
        pending_transactions: Cache<TxHash, TxStatus, hashbrown::hash_map::DefaultHashBuilder>,
        open_request_handle_metrics: Arc<OpenRequestHandleMetrics>,
    ) -> anyhow::Result<(Arc<Self>, AnyhowJoinHandle<()>)> {
        let (pending_tx_id_sender, pending_tx_id_receiver) = flume::unbounded();
        let (block_sender, block_receiver) = flume::unbounded::<BlockAndRpc>();

        let http_interval_sender = if http_client.is_some() {
            let (sender, receiver) = broadcast::channel(1);

            drop(receiver);

            // TODO: what interval? follow a websocket also? maybe by watching synced connections with a timeout. will need debounce
            let mut interval = interval(Duration::from_secs(13));
            interval.set_missed_tick_behavior(MissedTickBehavior::Delay);

            let sender = Arc::new(sender);

            let f = {
                let sender = sender.clone();

                async move {
                    loop {
                        // TODO: every time a head_block arrives (with a small delay for known slow servers), or on the interval.
                        interval.tick().await;

                        // // trace!("http interval ready");

                        // errors are okay. they mean that all receivers have been dropped
                        let _ = sender.send(());
                    }
                }
            };

            // TODO: do something with this handle?
            tokio::spawn(f);

            Some(sender)
        } else {
            None
        };

        // turn configs into connections (in parallel)
        // TODO: move this into a helper function. then we can use it when configs change (will need a remove function too)
        let spawn_handles: Vec<_> = server_configs
            .into_iter()
            .filter_map(|(server_name, server_config)| {
                if server_config.disabled {
                    return None;
                }

                let db_conn = db_conn.clone();
                let http_client = http_client.clone();
                let redis_pool = redis_pool.clone();
                let http_interval_sender = http_interval_sender.clone();

                let block_sender = if head_block_sender.is_some() {
                    Some(block_sender.clone())
                } else {
                    None
                };

                let pending_tx_id_sender = Some(pending_tx_id_sender.clone());
                let block_map = block_map.clone();
                let open_request_handle_metrics = open_request_handle_metrics.clone();

                let handle = tokio::spawn(async move {
                    server_config
                        .spawn(
                            server_name,
                            db_conn,
                            redis_pool,
                            chain_id,
                            http_client,
                            http_interval_sender,
                            block_map,
                            block_sender,
                            pending_tx_id_sender,
                            open_request_handle_metrics,
                        )
                        .await
                });

                Some(handle)
            })
            .collect();

        // map of connection names to their connection
        let mut connections = HashMap::new();
        let mut handles = vec![];

        // TODO: futures unordered?
        for x in join_all(spawn_handles).await {
            // TODO: how should we handle errors here? one rpc being down shouldn't cause the program to exit
            match x {
                Ok(Ok((connection, handle))) => {
                    connections.insert(connection.name.clone(), connection);
                    handles.push(handle);
                }
                Ok(Err(err)) => {
                    // if we got an error here, it is not retryable
                    // TODO: include context about which connection failed
                    error!("Unable to create connection. err={:?}", err);
                }
                Err(err) => {
                    return Err(err.into());
                }
            }
        }

        // TODO: now this errors for private rpcs when we disable all!
        if connections.len() < min_head_rpcs {
            return Err(anyhow::anyhow!(
                "Only {}/{} connections! Add more connections or reduce min_head_rpcs.",
                connections.len(),
                min_head_rpcs
            ));
        }

        // TODO: safety check on sum soft limit

        let synced_connections = SyncedConnections::default();

        // TODO: max_capacity and time_to_idle from config
        // all block hashes are the same size, so no need for weigher
        let block_hashes = Cache::builder()
            .time_to_idle(Duration::from_secs(600))
            .max_capacity(10_000)
            .build_with_hasher(hashbrown::hash_map::DefaultHashBuilder::default());
        // all block numbers are the same size, so no need for weigher
        let block_numbers = Cache::builder()
            .time_to_idle(Duration::from_secs(600))
            .max_capacity(10_000)
            .build_with_hasher(hashbrown::hash_map::DefaultHashBuilder::default());

        let connections = Arc::new(Self {
            conns: connections,
            synced_connections: ArcSwap::new(Arc::new(synced_connections)),
            pending_transactions,
            block_hashes,
            block_numbers,
            blockchain_graphmap: Default::default(),
            min_sum_soft_limit,
            min_head_rpcs,
        });

        let authorization = Arc::new(Authorization::internal(db_conn.clone())?);

        let handle = {
            let connections = connections.clone();

            tokio::spawn(async move {
                // TODO: try_join_all with the other handles here
                connections
                    .subscribe(
                        authorization,
                        pending_tx_id_receiver,
                        block_receiver,
                        head_block_sender,
                        pending_tx_sender,
                    )
                    .await
            })
        };

        Ok((connections, handle))
    }

    pub fn get(&self, conn_name: &str) -> Option<&Arc<Web3Connection>> {
        self.conns.get(conn_name)
    }

    /// subscribe to blocks and transactions from all the backend rpcs.
    /// blocks are processed by all the `Web3Connection`s and then sent to the `block_receiver`
    /// transaction ids from all the `Web3Connection`s are deduplicated and forwarded to `pending_tx_sender`
    async fn subscribe(
        self: Arc<Self>,
        authorization: Arc<Authorization>,
        pending_tx_id_receiver: flume::Receiver<TxHashAndRpc>,
        block_receiver: flume::Receiver<BlockAndRpc>,
        head_block_sender: Option<watch::Sender<ArcBlock>>,
        pending_tx_sender: Option<broadcast::Sender<TxStatus>>,
    ) -> anyhow::Result<()> {
        let mut futures = vec![];

        // setup the transaction funnel
        // it skips any duplicates (unless they are being orphaned)
        // fetches new transactions from the notifying rpc
        // forwards new transacitons to pending_tx_receipt_sender
        if let Some(pending_tx_sender) = pending_tx_sender.clone() {
            let clone = self.clone();
            let authorization = authorization.clone();
            let handle = task::spawn(async move {
                // TODO: set up this future the same as the block funnel
                while let Ok((pending_tx_id, rpc)) = pending_tx_id_receiver.recv_async().await {
                    let f = clone.clone().process_incoming_tx_id(
                        authorization.clone(),
                        rpc,
                        pending_tx_id,
                        pending_tx_sender.clone(),
                    );
                    tokio::spawn(f);
                }

                Ok(())
            });

            futures.push(flatten_handle(handle));
        }

        // setup the block funnel
        if let Some(head_block_sender) = head_block_sender {
            let connections = Arc::clone(&self);
            let pending_tx_sender = pending_tx_sender.clone();

            let handle = task::Builder::default()
                .name("process_incoming_blocks")
                .spawn(async move {
                    connections
                        .process_incoming_blocks(
                            &authorization,
                            block_receiver,
                            head_block_sender,
                            pending_tx_sender,
                        )
                        .await
                })?;

            futures.push(flatten_handle(handle));
        }

        if futures.is_empty() {
            // no transaction or block subscriptions.

            let handle = task::Builder::default().name("noop").spawn(async move {
                loop {
                    sleep(Duration::from_secs(600)).await;
                    // TODO: "every interval, check that the provider is still connected"
                }
            })?;

            futures.push(flatten_handle(handle));
        }

        if let Err(e) = try_join_all(futures).await {
            error!("subscriptions over: {:?}", self);
            return Err(e);
        }

        info!("subscriptions over: {:?}", self);

        Ok(())
    }

    /// Send the same request to all the handles. Returning the most common success or most common error.
    pub async fn try_send_parallel_requests(
        &self,
        active_request_handles: Vec<OpenRequestHandle>,
        method: &str,
        params: Option<&serde_json::Value>,
        // TODO: remove this box once i figure out how to do the options
    ) -> Result<Box<RawValue>, ProviderError> {
        // TODO: if only 1 active_request_handles, do self.try_send_request?

        let responses = active_request_handles
            .into_iter()
            .map(|active_request_handle| async move {
                let result: Result<Box<RawValue>, _> = active_request_handle
                    .request(method, &json!(&params), Level::Error.into())
                    .await;
                result
            })
            .collect::<FuturesUnordered<_>>()
            .collect::<Vec<Result<Box<RawValue>, ProviderError>>>()
            .await;

        // TODO: Strings are not great keys, but we can't use RawValue or ProviderError as keys because they don't implement Hash or Eq
        let mut count_map: HashMap<String, Result<Box<RawValue>, ProviderError>> = HashMap::new();
        let mut counts: Counter<String> = Counter::new();
        let mut any_ok = false;
        for response in responses {
            // TODO: i think we need to do something smarter with provider error. we at least need to wrap it up as JSON
            // TODO: emit stats errors?
            let s = format!("{:?}", response);

            if count_map.get(&s).is_none() {
                if response.is_ok() {
                    any_ok = true;
                }

                count_map.insert(s.clone(), response);
            }

            counts.update([s].into_iter());
        }

        for (most_common, _) in counts.most_common_ordered() {
            let most_common = count_map.remove(&most_common).unwrap();

            if any_ok && most_common.is_err() {
                //  errors were more common, but we are going to skip them because we got an okay
                continue;
            } else {
                // return the most common
                return most_common;
            }
        }

        // TODO: what should we do if we get here? i don't think we will
        panic!("i don't think this is possible")
    }

    /// get the best available rpc server
    pub async fn best_synced_backend_connection(
        &self,
        authorization: &Arc<Authorization>,
        request_metadata: Option<&Arc<RequestMetadata>>,
        skip: &[Arc<Web3Connection>],
        min_block_needed: Option<&U64>,
    ) -> anyhow::Result<OpenRequestResult> {
        let mut earliest_retry_at = None;

        let usable_rpcs: Vec<Arc<Web3Connection>> = if let Some(min_block_needed) = min_block_needed
        {
            // need a potentially old block. check all the rpcs
            // TODO: we are going to be checking "has_block_data" a lot now
            self.conns
                .values()
                .filter(|x| !skip.contains(x))
                .filter(|x| x.has_block_data(min_block_needed))
                .cloned()
                .collect()
        } else {
            // need latest. filter the synced rpcs
            // TODO: double check has_block_data?
            self.synced_connections
                .load()
                .conns
                .iter()
                .filter(|x| !skip.contains(x))
                .cloned()
                .collect()
        };

        match usable_rpcs.len() {
            0 => {
                warn!(
                    "no rpcs @ {:?}: {:?} (skipped {:?})",
                    min_block_needed,
                    self.synced_connections.load(),
                    skip.iter().map(|x| &x.name).collect::<Vec<_>>()
                );
                // TODO: what should happen here? automatic retry?
                // TODO: more detailed error
                return Ok(OpenRequestResult::NotSynced);
            }
            1 => {
                let rpc = usable_rpcs.get(0).expect("len is 1");

                // TODO: try or wait for a request handle?
                let handle = rpc
                    .wait_for_request_handle(authorization, Duration::from_secs(60))
                    .await?;

                return Ok(OpenRequestResult::Handle(handle));
            }
            _ => {
                // anything else and we need to pick with a weighted random chooser
            }
        }

        let mut minimum = f64::MAX;

        // we sort on a bunch of values. cache them here so that we don't do this math multiple times.
        let available_request_map: HashMap<_, f64> = usable_rpcs
            .iter()
            .map(|rpc| {
                // TODO: are active requests what we want? do we want a counter for requests in the last second + any actives longer than that?
                // TODO: get active requests out of redis (that's definitely too slow)
                // TODO: do something with hard limit instead? (but that is hitting redis too much)
                let active_requests = rpc.active_requests() as f64;
                let soft_limit = rpc.soft_limit as f64 * rpc.weight;

                // TODO: maybe store weight as the percentile
                let available_requests = soft_limit - active_requests;

                trace!("available requests on {}: {}", rpc, available_requests);

                // under heavy load, it is possible for even our best server to be negative
                minimum = available_requests.min(minimum);

                (rpc.clone(), available_requests)
            })
            .collect();

        trace!("minimum available requests: {}", minimum);

        // weights can't have negative numbers. shift up if any are negative
        let available_request_map: HashMap<_, f64> = if minimum < 0.0 {
            available_request_map
                .into_iter()
                .map(|(rpc, weight)| {
                    // TODO: is simple addition the right way to shift everyone?
                    // TODO: probably want something non-linear
                    // minimum is negative, so we subtract
                    let x = weight - minimum;

                    (rpc, x)
                })
                .collect()
        } else {
            available_request_map
        };

        let sorted_rpcs = {
            if usable_rpcs.len() == 1 {
                vec![usable_rpcs.get(0).expect("there should be 1")]
            } else {
                let mut rng = thread_fast_rng::thread_fast_rng();

                // TODO: sort or weight the non-archive nodes to be first
                usable_rpcs
                    .choose_multiple_weighted(&mut rng, usable_rpcs.len(), |rpc| {
                        *available_request_map
                            .get(rpc)
                            .expect("rpc should always be in the weight map")
                    })
                    .unwrap()
                    .collect::<Vec<_>>()
            }
        };

        // now that the rpcs are sorted, try to get an active request handle for one of them
        for rpc in sorted_rpcs.iter() {
            // increment our connection counter
            match rpc.try_request_handle(authorization).await {
                Ok(OpenRequestResult::Handle(handle)) => {
                    // // trace!("next server on {:?}: {:?}", self, rpc);
                    return Ok(OpenRequestResult::Handle(handle));
                }
                Ok(OpenRequestResult::RetryAt(retry_at)) => {
                    earliest_retry_at = earliest_retry_at.min(Some(retry_at));
                }
                Ok(OpenRequestResult::NotSynced) => {
                    // TODO: log a warning?
                }
                Err(err) => {
                    // TODO: log a warning?
                    warn!("No request handle for {}. err={:?}", rpc, err)
                }
            }
        }

        match earliest_retry_at {
            None => {
                // none of the servers gave us a time to retry at
                if let Some(request_metadata) = request_metadata {
                    request_metadata.no_servers.fetch_add(1, Ordering::Release);
                }

                // we could return an error here, but maybe waiting a second will fix the problem
                // TODO: configurable max wait? the whole max request time, or just some portion?
                let handle = sorted_rpcs
                    .get(0)
                    .expect("at least 1 is available")
                    .wait_for_request_handle(authorization, Duration::from_secs(3))
                    .await?;

                Ok(OpenRequestResult::Handle(handle))
            }
            Some(earliest_retry_at) => {
                warn!("no servers on {:?}! {:?}", self, earliest_retry_at);

                if let Some(request_metadata) = request_metadata {
                    request_metadata.no_servers.fetch_add(1, Ordering::Release);
                }

                Ok(OpenRequestResult::RetryAt(earliest_retry_at))
            }
        }
    }

    /// get all rpc servers that are not rate limited
    /// returns servers even if they aren't in sync. This is useful for broadcasting signed transactions
    // TODO: better type on this that can return an anyhow::Result
    pub async fn all_backend_connections(
        &self,
        authorization: &Arc<Authorization>,
        block_needed: Option<&U64>,
    ) -> Result<Vec<OpenRequestHandle>, Option<Instant>> {
        let mut earliest_retry_at = None;
        // TODO: with capacity?
        let mut selected_rpcs = vec![];

        for connection in self.conns.values() {
            if let Some(block_needed) = block_needed {
                if !connection.has_block_data(block_needed) {
                    continue;
                }
            }

            // check rate limits and increment our connection counter
            match connection.try_request_handle(authorization).await {
                Ok(OpenRequestResult::RetryAt(retry_at)) => {
                    // this rpc is not available. skip it
                    earliest_retry_at = earliest_retry_at.min(Some(retry_at));
                }
                Ok(OpenRequestResult::Handle(handle)) => selected_rpcs.push(handle),
                Ok(OpenRequestResult::NotSynced) => {
                    warn!("no request handle for {}", connection)
                }
                Err(err) => {
                    warn!(
                        "error getting request handle for {}. err={:?}",
                        connection, err
                    )
                }
            }
        }

        if !selected_rpcs.is_empty() {
            return Ok(selected_rpcs);
        }

        // return the earliest retry_after (if no rpcs are synced, this will be None)
        Err(earliest_retry_at)
    }

    /// be sure there is a timeout on this or it might loop forever
    pub async fn try_send_best_upstream_server(
        &self,
        authorization: &Arc<Authorization>,
        request: JsonRpcRequest,
        request_metadata: Option<&Arc<RequestMetadata>>,
        min_block_needed: Option<&U64>,
    ) -> anyhow::Result<JsonRpcForwardedResponse> {
        let mut skip_rpcs = vec![];

        // TODO: maximum retries? right now its the total number of servers
        loop {
            if skip_rpcs.len() == self.conns.len() {
                // no servers to try
                break;
            }
            match self
                .best_synced_backend_connection(
                    authorization,
                    request_metadata,
                    &skip_rpcs,
                    min_block_needed,
                )
                .await?
            {
                OpenRequestResult::Handle(active_request_handle) => {
                    // save the rpc in case we get an error and want to retry on another server
                    skip_rpcs.push(active_request_handle.clone_connection());

                    if let Some(request_metadata) = request_metadata {
                        request_metadata
                            .backend_requests
                            .fetch_add(1, Ordering::Acquire);
                    }

                    // TODO: get the log percent from the user data
                    let response_result = active_request_handle
                        .request(
                            &request.method,
                            &json!(request.params),
                            RequestErrorHandler::SaveReverts,
                        )
                        .await;

                    match JsonRpcForwardedResponse::try_from_response_result(
                        response_result,
                        request.id.clone(),
                    ) {
                        Ok(response) => {
                            if let Some(error) = &response.error {
                                // // trace!(?response, "rpc error");

                                if let Some(request_metadata) = request_metadata {
                                    request_metadata
                                        .error_response
                                        .store(true, Ordering::Release);
                                }

                                // some errors should be retried on other nodes
                                if error.code == -32000 {
                                    let error_msg = error.message.as_str();

                                    // TODO: regex?
                                    let retry_prefixes = [
                                        "header not found",
                                        "header for hash not found",
                                        "missing trie node",
                                        "node not started",
                                        "RPC timeout",
                                    ];
                                    for retry_prefix in retry_prefixes {
                                        if error_msg.starts_with(retry_prefix) {
                                            continue;
                                        }
                                    }
                                }
                            } else {
                                // // trace!(?response, "rpc success");
                            }

                            return Ok(response);
                        }
                        Err(err) => {
                            let rpc = skip_rpcs
                                .last()
                                .expect("there must have been a provider if we got an error");

                            warn!(
                                "Backend server error on {}! Retrying on another. err={:?}",
                                rpc, err
                            );

                            // TODO: sleep how long? until synced_connections changes or rate limits are available
                            // sleep(Duration::from_millis(100)).await;

                            continue;
                        }
                    }
                }
                OpenRequestResult::RetryAt(retry_at) => {
                    // TODO: move this to a helper function
                    // sleep (TODO: with a lock?) until our rate limits should be available
                    // TODO: if a server catches up sync while we are waiting, we could stop waiting
                    warn!("All rate limits exceeded. Sleeping untill {:?}", retry_at);

                    // TODO: have a separate column for rate limited?
                    if let Some(request_metadata) = request_metadata {
                        request_metadata.no_servers.fetch_add(1, Ordering::Release);
                    }

                    sleep_until(retry_at).await;

                    continue;
                }
                OpenRequestResult::NotSynced => {
                    if let Some(request_metadata) = request_metadata {
                        request_metadata.no_servers.fetch_add(1, Ordering::Release);
                    }

                    // TODO: subscribe to something on synced connections. maybe it should just be a watch channel
                    sleep(Duration::from_millis(200)).await;

                    continue;
                }
            }
        }

        // TODO: do we need this here, or do we do it somewhere else?
        if let Some(request_metadata) = request_metadata {
            request_metadata
                .error_response
                .store(true, Ordering::Release);
        }

        warn!("No synced servers! {:?}", self.synced_connections.load());

        // TODO: what error code? 502?
        Err(anyhow::anyhow!("all {} tries exhausted", skip_rpcs.len()))
    }

    /// be sure there is a timeout on this or it might loop forever

    pub async fn try_send_all_upstream_servers(
        &self,
        authorization: &Arc<Authorization>,
        request: JsonRpcRequest,
        request_metadata: Option<Arc<RequestMetadata>>,
        block_needed: Option<&U64>,
    ) -> anyhow::Result<JsonRpcForwardedResponse> {
        loop {
            match self
                .all_backend_connections(authorization, block_needed)
                .await
            {
                Ok(active_request_handles) => {
                    // TODO: benchmark this compared to waiting on unbounded futures
                    // TODO: do something with this handle?
                    // TODO: this is not working right. simplify

                    if let Some(request_metadata) = request_metadata {
                        request_metadata
                            .backend_requests
                            .fetch_add(active_request_handles.len() as u64, Ordering::Release);
                    }

                    let quorum_response = self
                        .try_send_parallel_requests(
                            active_request_handles,
                            request.method.as_ref(),
                            request.params.as_ref(),
                        )
                        .await?;

                    let response = JsonRpcForwardedResponse {
                        jsonrpc: "2.0".to_string(),
                        id: request.id,
                        result: Some(quorum_response),
                        error: None,
                    };

                    return Ok(response);
                }
                Err(None) => {
                    warn!("No servers in sync on {:?}! Retrying", self);

                    if let Some(request_metadata) = &request_metadata {
                        request_metadata.no_servers.fetch_add(1, Ordering::Release);
                    }

                    // TODO: i don't think this will ever happen
                    // TODO: return a 502? if it does?
                    // return Err(anyhow::anyhow!("no available rpcs!"));
                    // TODO: sleep how long?
                    // TODO: subscribe to something in SyncedConnections instead
                    sleep(Duration::from_millis(200)).await;

                    continue;
                }
                Err(Some(retry_at)) => {
                    // TODO: move this to a helper function
                    // sleep (TODO: with a lock?) until our rate limits should be available
                    // TODO: if a server catches up sync while we are waiting, we could stop waiting
                    warn!("All rate limits exceeded. Sleeping");

                    if let Some(request_metadata) = &request_metadata {
                        request_metadata.no_servers.fetch_add(1, Ordering::Release);
                    }

                    sleep_until(retry_at).await;

                    continue;
                }
            }
        }
    }
}

impl fmt::Debug for Web3Connections {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        // TODO: the default formatter takes forever to write. this is too quiet though
        f.debug_struct("Web3Connections")
            .field("conns", &self.conns)
            .finish_non_exhaustive()
    }
}

impl Serialize for Web3Connections {
    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
    where
        S: Serializer,
    {
        let mut state = serializer.serialize_struct("Web3Connections", 6)?;

        let conns: Vec<&Web3Connection> = self.conns.values().map(|x| x.as_ref()).collect();
        state.serialize_field("conns", &conns)?;

        let synced_connections = &**self.synced_connections.load();
        state.serialize_field("synced_connections", synced_connections)?;

        self.block_hashes.sync();
        self.block_numbers.sync();
        state.serialize_field("block_hashes_count", &self.block_hashes.entry_count())?;
        state.serialize_field("block_hashes_size", &self.block_hashes.weighted_size())?;
        state.serialize_field("block_numbers_count", &self.block_numbers.entry_count())?;
        state.serialize_field("block_numbers_size", &self.block_numbers.weighted_size())?;
        state.end()
    }
}

mod tests {
    #![allow(unused_imports)]
    use std::time::{SystemTime, UNIX_EPOCH};

    // TODO: why is this allow needed? does tokio::test get in the way somehow?
    use super::*;
    use crate::rpcs::{blockchain::BlockId, provider::Web3Provider};
    use ethers::types::{Block, U256};
    use log::{trace, LevelFilter};
    use parking_lot::RwLock;

    #[tokio::test]
    async fn test_server_selection_by_height() {
        // TODO: do this better. can test_env_logger and tokio test be stacked?
        let _ = env_logger::builder()
            .filter_level(LevelFilter::Error)
            .filter_module("web3_proxy", LevelFilter::Trace)
            .is_test(true)
            .try_init();

        let now: U256 = SystemTime::now()
            .duration_since(UNIX_EPOCH)
            .unwrap()
            .as_secs()
            .into();

        let lagged_block = Block {
            hash: Some(H256::random()),
            number: Some(0.into()),
            timestamp: now - 1,
            ..Default::default()
        };

        let head_block = Block {
            hash: Some(H256::random()),
            number: Some(1.into()),
            parent_hash: lagged_block.hash.unwrap(),
            timestamp: now,
            ..Default::default()
        };

        // TODO: write a impl From for Block -> BlockId?
        let lagged_block_id = BlockId {
            hash: lagged_block.hash.unwrap(),
            num: lagged_block.number.unwrap(),
        };
        let head_block_id = BlockId {
            hash: head_block.hash.unwrap(),
            num: head_block.number.unwrap(),
        };

        let lagged_block = Arc::new(lagged_block);
        let head_block = Arc::new(head_block);

        let block_data_limit = u64::MAX;

        let head_rpc = Web3Connection {
            name: "synced".to_string(),
            display_name: None,
            url: "ws://example.com/synced".to_string(),
            http_client: None,
            active_requests: 0.into(),
            frontend_requests: 0.into(),
            internal_requests: 0.into(),
            provider: AsyncRwLock::new(Some(Arc::new(Web3Provider::Mock))),
            hard_limit: None,
            soft_limit: 1_000,
            block_data_limit: block_data_limit.into(),
            weight: 100.0,
            head_block_id: RwLock::new(Some(head_block_id)),
            open_request_handle_metrics: Arc::new(Default::default()),
        };

        let lagged_rpc = Web3Connection {
            name: "lagged".to_string(),
            display_name: None,
            url: "ws://example.com/lagged".to_string(),
            http_client: None,
            active_requests: 0.into(),
            frontend_requests: 0.into(),
            internal_requests: 0.into(),
            provider: AsyncRwLock::new(Some(Arc::new(Web3Provider::Mock))),
            hard_limit: None,
            soft_limit: 1_000,
            block_data_limit: block_data_limit.into(),
            weight: 100.0,
            head_block_id: RwLock::new(Some(lagged_block_id)),
            open_request_handle_metrics: Arc::new(Default::default()),
        };

        assert!(head_rpc.has_block_data(&lagged_block.number.unwrap()));
        assert!(head_rpc.has_block_data(&head_block.number.unwrap()));

        assert!(lagged_rpc.has_block_data(&lagged_block.number.unwrap()));
        assert!(!lagged_rpc.has_block_data(&head_block.number.unwrap()));

        let head_rpc = Arc::new(head_rpc);
        let lagged_rpc = Arc::new(lagged_rpc);

        let conns = HashMap::from([
            (head_rpc.name.clone(), head_rpc.clone()),
            (lagged_rpc.name.clone(), lagged_rpc.clone()),
        ]);

        let conns = Web3Connections {
            conns,
            synced_connections: Default::default(),
            pending_transactions: Cache::builder()
                .max_capacity(10_000)
                .build_with_hasher(hashbrown::hash_map::DefaultHashBuilder::default()),
            block_hashes: Cache::builder()
                .max_capacity(10_000)
                .build_with_hasher(hashbrown::hash_map::DefaultHashBuilder::default()),
            block_numbers: Cache::builder()
                .max_capacity(10_000)
                .build_with_hasher(hashbrown::hash_map::DefaultHashBuilder::default()),
            blockchain_graphmap: Default::default(),
            min_head_rpcs: 1,
            min_sum_soft_limit: 1,
        };

        let authorization = Arc::new(Authorization::internal(None).unwrap());

        let (head_block_sender, _head_block_receiver) =
            watch::channel::<ArcBlock>(Default::default());
        let mut connection_heads = HashMap::new();

        // process None so that
        conns
            .process_block_from_rpc(
                &authorization,
                &mut connection_heads,
                None,
                lagged_rpc.clone(),
                &head_block_sender,
                &None,
            )
            .await
            .unwrap();
        conns
            .process_block_from_rpc(
                &authorization,
                &mut connection_heads,
                None,
                head_rpc.clone(),
                &head_block_sender,
                &None,
            )
            .await
            .unwrap();

        // no head block because the rpcs haven't communicated through their channels
        assert!(conns.head_block_hash().is_none());

        // all_backend_connections gives everything regardless of sync status
        assert_eq!(
            conns
                .all_backend_connections(&authorization, None)
                .await
                .unwrap()
                .len(),
            2
        );

        // best_synced_backend_connection requires servers to be synced with the head block
        let x = conns
            .best_synced_backend_connection(&authorization, None, &[], None)
            .await
            .unwrap();

        dbg!(&x);

        assert!(matches!(x, OpenRequestResult::NotSynced));

        // add lagged blocks to the conns. both servers should be allowed
        conns.save_block(&lagged_block, true).await.unwrap();

        conns
            .process_block_from_rpc(
                &authorization,
                &mut connection_heads,
                Some(lagged_block.clone()),
                lagged_rpc,
                &head_block_sender,
                &None,
            )
            .await
            .unwrap();
        conns
            .process_block_from_rpc(
                &authorization,
                &mut connection_heads,
                Some(lagged_block.clone()),
                head_rpc.clone(),
                &head_block_sender,
                &None,
            )
            .await
            .unwrap();

        assert_eq!(conns.num_synced_rpcs(), 2);

        // add head block to the conns. lagged_rpc should not be available
        conns.save_block(&head_block, true).await.unwrap();

        conns
            .process_block_from_rpc(
                &authorization,
                &mut connection_heads,
                Some(head_block.clone()),
                head_rpc,
                &head_block_sender,
                &None,
            )
            .await
            .unwrap();

        assert_eq!(conns.num_synced_rpcs(), 1);

        assert!(matches!(
            conns
                .best_synced_backend_connection(&authorization, None, &[], None)
                .await,
            Ok(OpenRequestResult::Handle(_))
        ));

        assert!(matches!(
            conns
                .best_synced_backend_connection(&authorization, None, &[], Some(&0.into()))
                .await,
            Ok(OpenRequestResult::Handle(_))
        ));

        assert!(matches!(
            conns
                .best_synced_backend_connection(&authorization, None, &[], Some(&1.into()))
                .await,
            Ok(OpenRequestResult::Handle(_))
        ));

        // future block should not get a handle
        assert!(matches!(
            conns
                .best_synced_backend_connection(&authorization, None, &[], Some(&2.into()))
                .await,
            Ok(OpenRequestResult::NotSynced)
        ));
    }

    #[tokio::test]
    async fn test_server_selection_by_archive() {
        // TODO: do this better. can test_env_logger and tokio test be stacked?
        let _ = env_logger::builder()
            .filter_level(LevelFilter::Error)
            .filter_module("web3_proxy", LevelFilter::Trace)
            .is_test(true)
            .try_init();

        let now: U256 = SystemTime::now()
            .duration_since(UNIX_EPOCH)
            .unwrap()
            .as_secs()
            .into();

        let head_block: Block<TxHash> = Block {
            hash: Some(H256::random()),
            number: Some(1_000_000.into()),
            parent_hash: H256::random(),
            timestamp: now,
            ..Default::default()
        };

        // TODO: write a impl From for Block -> BlockId?
        let head_block_id = BlockId {
            hash: head_block.hash.unwrap(),
            num: head_block.number.unwrap(),
        };

        let head_block = Arc::new(head_block);

        let pruned_rpc = Web3Connection {
            name: "pruned".to_string(),
            display_name: None,
            url: "ws://example.com/pruned".to_string(),
            http_client: None,
            active_requests: 0.into(),
            frontend_requests: 0.into(),
            internal_requests: 0.into(),
            provider: AsyncRwLock::new(Some(Arc::new(Web3Provider::Mock))),
            hard_limit: None,
            soft_limit: 3_000,
            block_data_limit: 64.into(),
            weight: 1.0,
            head_block_id: RwLock::new(Some(head_block_id.clone())),
            open_request_handle_metrics: Arc::new(Default::default()),
        };

        let archive_rpc = Web3Connection {
            name: "archive".to_string(),
            display_name: None,
            url: "ws://example.com/archive".to_string(),
            http_client: None,
            active_requests: 0.into(),
            frontend_requests: 0.into(),
            internal_requests: 0.into(),
            provider: AsyncRwLock::new(Some(Arc::new(Web3Provider::Mock))),
            hard_limit: None,
            soft_limit: 1_000,
            block_data_limit: u64::MAX.into(),
            // TODO: does weight = 0 work?
            weight: 0.01,
            head_block_id: RwLock::new(Some(head_block_id)),
            open_request_handle_metrics: Arc::new(Default::default()),
        };

        assert!(pruned_rpc.has_block_data(&head_block.number.unwrap()));
        assert!(archive_rpc.has_block_data(&head_block.number.unwrap()));
        assert!(!pruned_rpc.has_block_data(&1.into()));
        assert!(archive_rpc.has_block_data(&1.into()));

        let pruned_rpc = Arc::new(pruned_rpc);
        let archive_rpc = Arc::new(archive_rpc);

        let conns = HashMap::from([
            (pruned_rpc.name.clone(), pruned_rpc.clone()),
            (archive_rpc.name.clone(), archive_rpc.clone()),
        ]);

        let conns = Web3Connections {
            conns,
            synced_connections: Default::default(),
            pending_transactions: Cache::builder()
                .max_capacity(10)
                .build_with_hasher(hashbrown::hash_map::DefaultHashBuilder::default()),
            block_hashes: Cache::builder()
                .max_capacity(10)
                .build_with_hasher(hashbrown::hash_map::DefaultHashBuilder::default()),
            block_numbers: Cache::builder()
                .max_capacity(10)
                .build_with_hasher(hashbrown::hash_map::DefaultHashBuilder::default()),
            blockchain_graphmap: Default::default(),
            min_head_rpcs: 1,
            min_sum_soft_limit: 3_000,
        };

        let authorization = Arc::new(Authorization::internal(None).unwrap());

        let (head_block_sender, _head_block_receiver) =
            watch::channel::<ArcBlock>(Default::default());
        let mut connection_heads = HashMap::new();

        conns
            .process_block_from_rpc(
                &authorization,
                &mut connection_heads,
                Some(head_block.clone()),
                pruned_rpc.clone(),
                &head_block_sender,
                &None,
            )
            .await
            .unwrap();
        conns
            .process_block_from_rpc(
                &authorization,
                &mut connection_heads,
                Some(head_block.clone()),
                archive_rpc.clone(),
                &head_block_sender,
                &None,
            )
            .await
            .unwrap();

        assert_eq!(conns.num_synced_rpcs(), 2);

        // best_synced_backend_connection requires servers to be synced with the head block
        let best_head_server = conns
            .best_synced_backend_connection(&authorization, None, &[], head_block.number.as_ref())
            .await;

        assert!(matches!(
            best_head_server.unwrap(),
            OpenRequestResult::Handle(_)
        ));

        let best_archive_server = conns
            .best_synced_backend_connection(&authorization, None, &[], Some(&1.into()))
            .await;

        match best_archive_server {
            Ok(OpenRequestResult::Handle(x)) => {
                assert_eq!(x.clone_connection().name, "archive".to_string())
            }
            x => {
                error!("unexpected result: {:?}", x);
            }
        }
    }
}