Do not double compress transaction update messages

Author: joshua-spacetime

crates/bench/benches/subscription.rs

@@ -138,7 +138,6 @@ fn eval(c: &mut Criterion) {
                     &plans,
                     table_id,
                     table_name.clone(),
-                    Compression::None,
                     &tx,
                     TableUpdateType::Subscribe,
                 )))

crates/core/src/host/module_host.rs

@@ -128,13 +128,17 @@ impl UpdatesRelValue<'_> {
         !(self.deletes.is_empty() && self.inserts.is_empty())
     }
 
-    pub fn encode<F: WebsocketFormat>(&self, compression: Compression) -> (F::QueryUpdate, u64, usize) {
+    pub fn encode<F: WebsocketFormat>(&self) -> (F::QueryUpdate, u64, usize) {
         let (deletes, nr_del) = F::encode_list(self.deletes.iter());
         let (inserts, nr_ins) = F::encode_list(self.inserts.iter());
         let num_rows = nr_del + nr_ins;
         let num_bytes = deletes.num_bytes() + inserts.num_bytes();
         let qu = QueryUpdate { deletes, inserts };
-        let cqu = F::into_query_update(qu, compression);
+        // We don't compress individual table updates.
+        // Previously we were, but the benefits, if any, were unclear.
+        // Note, each message is still compressed before being sent to clients,
+        // but we no longer have to hold a tx lock when doing so.
+        let cqu = F::into_query_update(qu, Compression::None);
         (cqu, num_rows, num_bytes)
     }
 }

crates/core/src/subscription/mod.rs

@@ -115,7 +115,6 @@ pub fn collect_table_update<Tx, F>(
     plan_fragments: &[PipelinedProject],
     table_id: TableId,
     table_name: Box<str>,
-    comp: Compression,
     tx: &Tx,
     update_type: TableUpdateType,
 ) -> Result<(TableUpdate<F>, ExecutionMetrics)>
@@ -135,15 +134,17 @@ where
                 inserts: empty,
             },
         };
-        let update = F::into_query_update(qu, comp);
+        // We will compress the outer server message,
+        // after we release the tx lock.
+        // There's no need to compress the inner table update too.
+        let update = F::into_query_update(qu, Compression::None);
         (TableUpdate::new(table_id, table_name, (update, num_rows)), metrics)
     })
 }
 
 /// Execute a collection of subscription queries in parallel
 pub fn execute_plans<Tx, F>(
     plans: &[Arc<Plan>],
-    comp: Compression,
     tx: &Tx,
     update_type: TableUpdateType,
 ) -> Result<(DatabaseUpdate<F>, ExecutionMetrics), DBError>
@@ -160,7 +161,7 @@ where
                 .clone()
                 .optimize()
                 .map(|plan| (sql, PipelinedProject::from(plan)))
-                .and_then(|(_, plan)| collect_table_update(&[plan], table_id, table_name.into(), comp, tx, update_type))
+                .and_then(|(_, plan)| collect_table_update(&[plan], table_id, table_name.into(), tx, update_type))
                 .map_err(|err| DBError::WithSql {
                     sql: sql.into(),
                     error: Box::new(DBError::Other(err)),

crates/core/src/subscription/module_subscription_actor.rs

@@ -23,8 +23,8 @@ use crate::worker_metrics::WORKER_METRICS;
 use parking_lot::RwLock;
 use prometheus::IntGauge;
 use spacetimedb_client_api_messages::websocket::{
-    self as ws, BsatnFormat, Compression, FormatSwitch, JsonFormat, SubscribeMulti, SubscribeSingle, TableUpdate,
-    Unsubscribe, UnsubscribeMulti,
+    self as ws, BsatnFormat, FormatSwitch, JsonFormat, SubscribeMulti, SubscribeSingle, TableUpdate, Unsubscribe,
+    UnsubscribeMulti,
 };
 use spacetimedb_execution::pipelined::PipelinedProject;
 use spacetimedb_expr::check::parse_and_type_sub;
@@ -186,34 +186,10 @@ impl ModuleSubscriptions {
         let tx = DeltaTx::from(tx);
 
         Ok(match sender.config.protocol {
-            Protocol::Binary => {
-                collect_table_update(
-                    &plans,
-                    table_id,
-                    table_name.into(),
-                    // We will compress the outer server message,
-                    // after we release the tx lock.
-                    // There's no need to compress the inner table update too.
-                    Compression::None,
-                    &tx,
-                    update_type,
-                )
-                .map(|(table_update, metrics)| (FormatSwitch::Bsatn(table_update), metrics))
-            }
-            Protocol::Text => {
-                collect_table_update(
-                    &plans,
-                    table_id,
-                    table_name.into(),
-                    // We will compress the outer server message,
-                    // after we release the tx lock,
-                    // There's no need to compress the inner table update too.
-                    Compression::None,
-                    &tx,
-                    update_type,
-                )
-                .map(|(table_update, metrics)| (FormatSwitch::Json(table_update), metrics))
-            }
+            Protocol::Binary => collect_table_update(&plans, table_id, table_name.into(), &tx, update_type)
+                .map(|(table_update, metrics)| (FormatSwitch::Bsatn(table_update), metrics)),
+            Protocol::Text => collect_table_update(&plans, table_id, table_name.into(), &tx, update_type)
+                .map(|(table_update, metrics)| (FormatSwitch::Json(table_update), metrics)),
         }?)
     }
 
@@ -240,27 +216,11 @@ impl ModuleSubscriptions {
         let tx = DeltaTx::from(tx);
         match sender.config.protocol {
             Protocol::Binary => {
-                let (update, metrics) = execute_plans(
-                    queries,
-                    // We will compress the outer server message,
-                    // after we release the tx lock.
-                    // There's no need to compress the inner table updates too.
-                    Compression::None,
-                    &tx,
-                    update_type,
-                )?;
+                let (update, metrics) = execute_plans(queries, &tx, update_type)?;
                 Ok((FormatSwitch::Bsatn(update), metrics))
             }
             Protocol::Text => {
-                let (update, metrics) = execute_plans(
-                    queries,
-                    // We will compress the outer server message,
-                    // after we release the tx lock.
-                    // There's no need to compress the inner table updates too.
-                    Compression::None,
-                    &tx,
-                    update_type,
-                )?;
+                let (update, metrics) = execute_plans(queries, &tx, update_type)?;
                 Ok((FormatSwitch::Json(update), metrics))
             }
         }
@@ -650,26 +610,10 @@ impl ModuleSubscriptions {
 
         let tx = DeltaTx::from(&*tx);
         let (database_update, metrics) = match sender.config.protocol {
-            Protocol::Binary => execute_plans(
-                &queries,
-                // We will compress the outer server message,
-                // after we release the tx lock.
-                // There's no need to compress the inner table updates too.
-                Compression::None,
-                &tx,
-                TableUpdateType::Subscribe,
-            )
-            .map(|(table_update, metrics)| (FormatSwitch::Bsatn(table_update), metrics))?,
-            Protocol::Text => execute_plans(
-                &queries,
-                // We will compress the outer server message,
-                // after we release the tx lock.
-                // There's no need to compress the inner table updates too.
-                Compression::None,
-                &tx,
-                TableUpdateType::Subscribe,
-            )
-            .map(|(table_update, metrics)| (FormatSwitch::Json(table_update), metrics))?,
+            Protocol::Binary => execute_plans(&queries, &tx, TableUpdateType::Subscribe)
+                .map(|(table_update, metrics)| (FormatSwitch::Bsatn(table_update), metrics))?,
+            Protocol::Text => execute_plans(&queries, &tx, TableUpdateType::Subscribe)
+                .map(|(table_update, metrics)| (FormatSwitch::Json(table_update), metrics))?,
         };
 
         record_exec_metrics(

crates/core/src/subscription/module_subscription_manager.rs

@@ -16,7 +16,7 @@ use hashbrown::{HashMap, HashSet};
 use itertools::Itertools;
 use rayon::iter::{IntoParallelIterator, ParallelIterator};
 use spacetimedb_client_api_messages::websocket::{
-    BsatnFormat, CompressableQueryUpdate, Compression, FormatSwitch, JsonFormat, QueryId, QueryUpdate, WebsocketFormat,
+    BsatnFormat, CompressableQueryUpdate, FormatSwitch, JsonFormat, QueryId, QueryUpdate, WebsocketFormat,
 };
 use spacetimedb_data_structures::map::{Entry, IntMap};
 use spacetimedb_lib::metrics::ExecutionMetrics;
@@ -817,26 +817,37 @@ impl SubscriptionManager {
                 .fold(FoldState::default, |mut acc, (qstate, plan)| {
                     let table_id = plan.subscribed_table_id();
                     let table_name = plan.subscribed_table_name();
-                    // Store at most one copy of the serialization to BSATN x Compression
-                    // and ditto for the "serialization" for JSON.
+                    // Store at most one copy for both the serialization to BSATN and JSON.
                     // Each subscriber gets to pick which of these they want,
-                    // but we only fill `ops_bin_{compression}` and `ops_json` at most once.
+                    // but we only fill `ops_bin_uncompressed` and `ops_json` at most once.
                     // The former will be `Some(_)` if some subscriber uses `Protocol::Binary`
                     // and the latter `Some(_)` if some subscriber uses `Protocol::Text`.
-                    let mut ops_bin_brotli: Option<(CompressableQueryUpdate<BsatnFormat>, _, _)> = None;
-                    let mut ops_bin_gzip: Option<(CompressableQueryUpdate<BsatnFormat>, _, _)> = None;
-                    let mut ops_bin_none: Option<(CompressableQueryUpdate<BsatnFormat>, _, _)> = None;
+                    //
+                    // Previously we were compressing each `QueryUpdate` within a `TransactionUpdate`.
+                    // The reason was simple - many clients can subscribe to the same query.
+                    // If we compress `TransactionUpdate`s independently for each client,
+                    // we could be doing a lot of redundant compression.
+                    //
+                    // However the risks associated with this approach include:
+                    //   1. We have to hold the tx lock when compressing
+                    //   2. A potentially worse compression ratio
+                    //   3. Extra decompression overhead on the client
+                    //
+                    // Because transaction processing is currently single-threaded,
+                    // the risks of holding the tx lock for longer than necessary,
+                    // as well as additional the message processing overhead on the client,
+                    // outweighed the benefit of reduced cpu with the former approach.
+                    let mut ops_bin_uncompressed: Option<(CompressableQueryUpdate<BsatnFormat>, _, _)> = None;
                     let mut ops_json: Option<(QueryUpdate<JsonFormat>, _, _)> = None;
 
                     fn memo_encode<F: WebsocketFormat>(
                         updates: &UpdatesRelValue<'_>,
-                        client: &ClientConnectionSender,
                         memory: &mut Option<(F::QueryUpdate, u64, usize)>,
                         metrics: &mut ExecutionMetrics,
                     ) -> (F::QueryUpdate, u64) {
                         let (update, num_rows, num_bytes) = memory
                             .get_or_insert_with(|| {
-                                let encoded = updates.encode::<F>(client.config.compression);
+                                let encoded = updates.encode::<F>();
                                 // The first time we insert into this map, we call encode.
                                 // This is when we serialize the rows to BSATN/JSON.
                                 // Hence this is where we increment `bytes_scanned`.
@@ -884,17 +895,11 @@ impl SubscriptionManager {
                                 let update = match client.config.protocol {
                                     Protocol::Binary => Bsatn(memo_encode::<BsatnFormat>(
                                         &delta_updates,
-                                        client,
-                                        match client.config.compression {
-                                            Compression::Brotli => &mut ops_bin_brotli,
-                                            Compression::Gzip => &mut ops_bin_gzip,
-                                            Compression::None => &mut ops_bin_none,
-                                        },
+                                        &mut ops_bin_uncompressed,
                                         &mut acc.metrics,
                                     )),
                                     Protocol::Text => Json(memo_encode::<JsonFormat>(
                                         &delta_updates,
-                                        client,
                                         &mut ops_json,
                                         &mut acc.metrics,
                                     )),