Add configuration option to set warmupParallelism parameter for connection pool.

README.md

@@ -72,6 +72,7 @@ Mono<Object> resultMono = Mono.usingWhen(pooledConnectionFactory.create(),
 | `registerJmx`                | Whether to register the pool to JMX.
 | `validationDepth`            | Validation depth used to validate an R2DBC connection. Defaults to `LOCAL`.
 | `validationQuery`            | Query that will be executed just before a connection is given to you from the pool to validate that the connection to the database is still alive.
+| `warmupParallelism`          | The concurrency level used when the allocator is subscribed to during the warmup phase. Default to `1`.
 
 All other properties are driver-specific.
 

src/main/java/io/r2dbc/pool/ConnectionPool.java

@@ -276,9 +276,15 @@ private InstrumentedPool<Connection> createConnectionPool(ConnectionPoolConfigur
                 .idleResourceReuseMruOrder(); // MRU to support eviction of idle
 
         if (maxSize == -1 || initialSize > 0) {
-            builder.sizeBetween(Math.max(configuration.getMinIdle(), initialSize), maxSize == -1 ? Integer.MAX_VALUE : maxSize);
+            builder.sizeBetween(
+                Math.max(configuration.getMinIdle(), initialSize),
+                maxSize == -1 ? Integer.MAX_VALUE : maxSize,
+                configuration.getWarmupParallelism());
         } else {
-            builder.sizeBetween(Math.max(configuration.getMinIdle(), initialSize), maxSize);
+            builder.sizeBetween(
+                Math.max(configuration.getMinIdle(), initialSize),
+                maxSize,
+                configuration.getWarmupParallelism());
         }
 
         Duration backgroundEvictionInterval = configuration.getBackgroundEvictionInterval();

src/main/java/io/r2dbc/pool/ConnectionPoolConfiguration.java

@@ -52,6 +52,11 @@ public final class ConnectionPoolConfiguration {
      */
     public static final Duration NO_TIMEOUT = Duration.ofMillis(-1);
 
+    /**
+     * Constant indicating the default parallelism used during connection pool warmup.
+     */
+    public static final int DEFAULT_WARMUP_PARALLELISM = 1;
+
     @Nullable
     private final Scheduler allocatorSubscribeOn;
 
@@ -99,12 +104,14 @@ public final class ConnectionPoolConfiguration {
     @Nullable
     private final String validationQuery;
 
+    private final int warmupParallelism;
+
     private ConnectionPoolConfiguration(@Nullable Scheduler allocatorSubscribeOn, int acquireRetry, @Nullable Duration backgroundEvictionInterval, ConnectionFactory connectionFactory, Clock clock, Consumer<PoolBuilder<Connection, ?
             extends PoolConfig<? extends Connection>>> customizer, int initialSize, int maxSize, int minIdle, Duration maxAcquireTime, Duration maxCreateConnectionTime, Duration maxIdleTime,
                                         Duration maxLifeTime, Duration maxValidationTime, PoolMetricsRecorder metricsRecorder, @Nullable String name,
                                         @Nullable Function<? super Connection, ? extends Publisher<Void>> postAllocate,
                                         @Nullable Function<? super Connection, ? extends Publisher<Void>> preRelease, boolean registerJmx, ValidationDepth validationDepth,
-                                        @Nullable String validationQuery) {
+                                        @Nullable String validationQuery, int warmupParallelism) {
         this.allocatorSubscribeOn = allocatorSubscribeOn;
         this.acquireRetry = acquireRetry;
         this.connectionFactory = Assert.requireNonNull(connectionFactory, "ConnectionFactory must not be null");
@@ -126,6 +133,7 @@ private ConnectionPoolConfiguration(@Nullable Scheduler allocatorSubscribeOn, in
         this.validationDepth = validationDepth;
         this.validationQuery = validationQuery;
         this.backgroundEvictionInterval = backgroundEvictionInterval;
+        this.warmupParallelism = warmupParallelism;
     }
 
     /**
@@ -237,6 +245,10 @@ String getValidationQuery() {
         return this.validationQuery;
     }
 
+    int getWarmupParallelism() {
+        return this.warmupParallelism;
+    }
+
     /**
      * A builder for {@link ConnectionPoolConfiguration} instances.
      * <p>
@@ -293,6 +305,8 @@ public static final class Builder {
 
         private ValidationDepth validationDepth = ValidationDepth.LOCAL;
 
+        private int warmupParallelism = DEFAULT_WARMUP_PARALLELISM;
+
         private Builder() {
         }
 
@@ -583,6 +597,23 @@ public Builder validationQuery(String validationQuery) {
             return this;
         }
 
+        /**
+         * Configure the concurrency level used when the allocator is subscribed to during the warmup phase.
+         *
+         * @param warmupParallelism Specifies the concurrency level used when the allocator is subscribed to during the warmup phase, if any.
+         * During warmup, resources that can be pre-allocated will be created eagerly, but at most {@code warmupParallelism} resources are
+         * subscribed to at the same time.
+         * @return this {@link Builder}
+         * @throws IllegalArgumentException if {@code warmupParallelism} is negative
+         */
+        public Builder warmupParallelism(int warmupParallelism) {
+            if (warmupParallelism < 0) {
+                throw new IllegalArgumentException("warmupParallelism must not be negative");
+            }
+            this.warmupParallelism = warmupParallelism;
+            return this;
+        }
+
         /**
          * Returns a configured {@link ConnectionPoolConfiguration}.
          *
@@ -596,7 +627,7 @@ public ConnectionPoolConfiguration build() {
                     this.clock, this.customizer, this.initialSize, this.maxSize, this.minIdle,
                     this.maxAcquireTime, this.maxCreateConnectionTime, this.maxIdleTime, this.maxLifeTime, this.maxValidationTime,
                     this.metricsRecorder, this.name, this.postAllocate, this.preRelease, this.registerJmx,
-                    this.validationDepth, this.validationQuery
+                    this.validationDepth, this.validationQuery, this.warmupParallelism
             );
         }
 

src/main/java/io/r2dbc/pool/PoolingConnectionFactoryProvider.java

@@ -149,6 +149,11 @@ public class PoolingConnectionFactoryProvider implements ConnectionFactoryProvid
      */
     public static final Option<ValidationDepth> VALIDATION_DEPTH = Option.valueOf("validationDepth");
 
+    /**
+     * WarmupParallelism {@link Option}.
+     */
+    public static final Option<Integer> WARMUP_PARALLELISM = Option.valueOf("warmupParallelism");
+
     private static final String COLON = ":";
 
     /**
@@ -210,6 +215,7 @@ static ConnectionPoolConfiguration buildConfiguration(ConnectionFactoryOptions c
         mapper.from(REGISTER_JMX).as(OptionMapper::toBoolean).to(builder::registerJmx);
         mapper.fromExact(VALIDATION_QUERY).to(builder::validationQuery);
         mapper.from(VALIDATION_DEPTH).as(validationDepth -> OptionMapper.toEnum(validationDepth, ValidationDepth.class)).to(builder::validationDepth);
+        mapper.from(WARMUP_PARALLELISM).as(OptionMapper::toInteger).to(builder::warmupParallelism);
 
         return builder.build();
     }

src/test/java/io/r2dbc/pool/ConnectionPoolConfigurationUnitTests.java

@@ -49,6 +49,7 @@ void configuration() {
             .maxSize(20)
             .name("bar")
             .registerJmx(true)
+            .warmupParallelism(99)
             .build();
 
         assertThat(configuration)
@@ -62,7 +63,8 @@ void configuration() {
             .hasFieldOrPropertyWithValue("initialSize", 2)
             .hasFieldOrPropertyWithValue("maxSize", 20)
             .hasFieldOrPropertyWithValue("name", "bar")
-            .hasFieldOrPropertyWithValue("registerJmx", true);
+            .hasFieldOrPropertyWithValue("registerJmx", true)
+            .hasFieldOrPropertyWithValue("warmupParallelism", 99);
     }
 
     @Test
@@ -82,7 +84,8 @@ void configurationDefaults() {
             .hasFieldOrPropertyWithValue("maxValidationTime", Duration.ofMillis(-1))
             .hasFieldOrPropertyWithValue("initialSize", 10)
             .hasFieldOrPropertyWithValue("maxSize", 10)
-            .hasFieldOrPropertyWithValue("registerJmx", false);
+            .hasFieldOrPropertyWithValue("registerJmx", false)
+            .hasFieldOrPropertyWithValue("warmupParallelism", 1);
     }
 
     @Test