Configurable connection pool reuse policy

service-config/src/main/java/com/palantir/conjure/java/api/config/service/PartialServiceConfiguration.java

@@ -92,6 +92,10 @@ public interface PartialServiceConfiguration {
     @JsonAlias("proxy-configuration")
     Optional<ProxyConfiguration> proxyConfiguration();
 
+    /** Reuse policy for connection pooling. If absent, uses client implementation default. */
+    @JsonAlias("pool-reuse-policy")
+    Optional<PoolReusePolicy> poolReusePolicy();
+
     static PartialServiceConfiguration of(List<String> uris, Optional<SslConfiguration> sslConfig) {
         return PartialServiceConfiguration.builder()
                 .uris(uris)

service-config/src/main/java/com/palantir/conjure/java/api/config/service/PoolReusePolicy.java

@@ -0,0 +1,26 @@
+/*
+ * (c) Copyright 2025 Palantir Technologies Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.palantir.conjure.java.api.config.service;
+
+public enum PoolReusePolicy {
+
+    /** Re-use as few connections as possible, making it possible for connections to become idle and expire. */
+    LIFO,
+
+    /** Re-use all connections equally, preventing them from becoming idle and expiring. */
+    FIFO,
+}

service-config/src/main/java/com/palantir/conjure/java/api/config/service/ServiceConfiguration.java

@@ -57,6 +57,8 @@ public interface ServiceConfiguration {
 
     Optional<ProxyConfiguration> proxy();
 
+    Optional<PoolReusePolicy> poolReusePolicy();
+
     static ImmutableServiceConfiguration.Builder builder() {
         return new Builder();
     }

service-config/src/main/java/com/palantir/conjure/java/api/config/service/ServiceConfigurationFactory.java

@@ -90,6 +90,7 @@ private ServiceConfiguration propagateDefaults(String serviceName, PartialServic
                 .enableHttp2(orElse(partial.enableHttp2(), services.defaultEnableHttp2()))
                 .fallbackToCommonNameVerification(orElse(
                         partial.fallbackToCommonNameVerification(), services.defaultFallbackToCommonNameVerification()))
+                .poolReusePolicy(orElse(partial.poolReusePolicy(), services.defaultPoolReusePolicy()))
                 .build();
     }
 

service-config/src/main/java/com/palantir/conjure/java/api/config/service/ServicesConfigBlock.java

@@ -105,6 +105,11 @@ public abstract class ServicesConfigBlock {
     @JsonAlias("fallback-to-common-name-verification")
     public abstract Optional<Boolean> defaultFallbackToCommonNameVerification();
 
+    /** Default global pool reuse policy. */
+    @JsonProperty("poolReusePolicy")
+    @JsonAlias("pool-reuse-policy")
+    public abstract Optional<PoolReusePolicy> defaultPoolReusePolicy();
+
     public static ServicesConfigBlock empty() {
         return ImmutableServicesConfigBlock.of();
     }

service-config/src/test/java/com/palantir/conjure/java/api/config/service/PartialServiceConfigurationTest.java

@@ -50,15 +50,15 @@ public void serDe() throws Exception {
                 + "\"maxNumRetries\":5,\"backoffSlotSize\":\"1 day\","
                 + "\"enableGcmCipherSuites\":null,\"enableHttp2\":null,\"fallbackToCommonNameVerification\":null,"
                 + "\"proxyConfiguration\":{\"hostAndPort\":\"host:80\",\"credentials\":null,"
-                + "\"type\":\"HTTP\"}}";
+                + "\"type\":\"HTTP\"},\"poolReusePolicy\":null}";
         String kebabCase = "{\"api-token\":\"bearerToken\",\"security\":"
                 + "{\"trust-store-path\":\"truststore.jks\",\"trust-store-type\":\"JKS\",\"key-store-path\":null,"
                 + "\"key-store-password\":null,\"key-store-type\":\"JKS\",\"key-store-key-alias\":null},"
                 + "\"connect-timeout\":\"1 day\",\"read-timeout\":\"1 day\",\"write-timeout\":\"1 day\","
                 + "\"max-num-retries\":5,\"backoff-slot-size\":\"1 day\","
                 + "\"uris\":[\"uri1\"],\"proxy-configuration\":{\"host-and-port\":\"host:80\",\"credentials\":null},"
                 + "\"enable-gcm-cipher-suites\":null,\"enable-http2\":null,"
-                + "\"fallback-to-common-name-verification\":null}";
+                + "\"fallback-to-common-name-verification\":null,\"pool-reuse-policy\":null}";
 
         assertThat(mapper.writeValueAsString(serialized)).isEqualTo(camelCase);
         assertThat(mapper.readValue(camelCase, PartialServiceConfiguration.class))
@@ -74,12 +74,12 @@ public void serDe_optional() throws Exception {
         String camelCase = "{\"apiToken\":null,\"security\":null,\"uris\":[],\"connectTimeout\":null,"
                 + "\"readTimeout\":null,\"writeTimeout\":null,\"maxNumRetries\":null,\"backoffSlotSize\":null,"
                 + "\"enableGcmCipherSuites\":null,\"enableHttp2\":null,\"fallbackToCommonNameVerification\":null,"
-                + "\"proxyConfiguration\":null}";
+                + "\"proxyConfiguration\":null,\"poolReusePolicy\":null}";
         String kebabCase = "{\"api-token\":null,\"security\":null,\"connect-timeout\":null,"
                 + "\"read-timeout\":null,\"write-timeout\":null,\"max-num-retries\":null,\"backoff-slot-size\":null,"
                 + "\"enable-gcm-cipher-suites\":null,\"enable-http2\":null,"
                 + "\"fallback-to-common-name-verification\":null,"
-                + "\"uris\":[],\"proxy-configuration\":null}";
+                + "\"uris\":[],\"proxy-configuration\":null,\"pool-reuse-policy\":null}";
 
         assertThat(ObjectMappers.newClientObjectMapper().writeValueAsString(serialized))
                 .isEqualTo(camelCase);

service-config/src/test/java/com/palantir/conjure/java/api/config/service/ServiceConfigurationFactoryTests.java

@@ -62,6 +62,8 @@ public final class ServiceConfigurationFactoryTests {
     private static final boolean enableGcm = false;
     private static final boolean defaultFallbackToCn = true;
     private static final boolean fallbackToCn = false;
+    private static final PoolReusePolicy poolReusePolicy = PoolReusePolicy.FIFO;
+    private static final PoolReusePolicy defaultPoolReusePolicy = PoolReusePolicy.LIFO;
 
     private final ObjectMapper mapper = new ObjectMapper(new YAMLFactory())
             .registerModule(new ShimJdk7Module())
@@ -136,12 +138,14 @@ public void testServiceSpecificConfigTrumpsDefaultConfig() {
                 .enableGcmCipherSuites(enableGcm)
                 .fallbackToCommonNameVerification(fallbackToCn)
                 .proxyConfiguration(proxy)
+                .poolReusePolicy(poolReusePolicy)
                 .build();
         ServicesConfigBlock services = ServicesConfigBlock.builder()
                 .putAllServices(ImmutableMap.of("service1", partial))
                 .defaultSecurity(defaultSecurity)
                 .defaultApiToken(defaultApiToken)
                 .defaultProxyConfiguration(defaultProxyConfiguration)
+                .defaultPoolReusePolicy(defaultPoolReusePolicy)
                 .defaultConnectTimeout(defaultConnectTimeout)
                 .defaultReadTimeout(defaultReadTimeout)
                 .defaultWriteTimeout(defaultWriteTimeout)
@@ -163,6 +167,7 @@ public void testServiceSpecificConfigTrumpsDefaultConfig() {
                 .enableGcmCipherSuites(enableGcm)
                 .fallbackToCommonNameVerification(fallbackToCn)
                 .proxy(proxy)
+                .poolReusePolicy(poolReusePolicy)
                 .build();
 
         assertThat(service).isEqualTo(expected);
@@ -198,18 +203,19 @@ public void serDe() throws Exception {
                 + "{\"service\":{\"apiToken\":null,\"security\":null,\"uris\":[\"uri\"],\"connectTimeout\":null,"
                 + "\"readTimeout\":null,\"writeTimeout\":null,\"maxNumRetries\":null,\"backoffSlotSize\":null,"
                 + "\"enableGcmCipherSuites\":null,\"enableHttp2\":null,\"fallbackToCommonNameVerification\":null,"
-                + "\"proxyConfiguration\":null}},\"proxyConfiguration\":"
-                + "{\"hostAndPort\":\"host:80\",\"credentials\":null,\"type\":\"HTTP\"},\"connectTimeout\":\"1 day\","
+                + "\"proxyConfiguration\":null,\"poolReusePolicy\":null}},\"proxyConfiguration\":{\"hostAndPort\":"
+                + "\"host:80\",\"credentials\":null,\"type\":\"HTTP\"},\"connectTimeout\":\"1 day\","
                 + "\"readTimeout\":\"1 day\",\"writeTimeout\":\"1 day\",\"backoffSlotSize\":\"1 day\","
-                + "\"enableGcmCipherSuites\":null,\"enableHttp2\":null,\"fallbackToCommonNameVerification\":null}";
+                + "\"enableGcmCipherSuites\":null,\"enableHttp2\":null,\"fallbackToCommonNameVerification\":null,"
+                + "\"poolReusePolicy\":null}";
         String kebabCase = "{\"api-token\":\"bearerToken\",\"security\":"
                 + "{\"trust-store-path\":\"truststore.jks\",\"trust-store-type\":\"JKS\",\"key-store-path\":null,"
                 + "\"key-store-password\":null,\"key-store-type\":\"JKS\",\"key-store-key-alias\":null},\"services\":"
                 + "{\"service\":{\"apiToken\":null,\"security\":null,\"connect-timeout\":null,\"read-timeout\":null,"
                 + "\"write-timeout\":null,\"max-num-retries\":null,\"backoffSlotSize\":null,\"uris\":[\"uri\"],"
                 + "\"enable-gcm-cipher-suites\":null,\"enable-http2\":null,"
                 + "\"fallback-to-common-name-verification\":null,"
-                + "\"proxy-configuration\":null}},\"proxy-configuration\":"
+                + "\"proxy-configuration\":null,\"pool-reuse-policy\":null}},\"proxy-configuration\":"
                 + "{\"host-and-port\":\"host:80\",\"credentials\":null},\"connect-timeout\":\"1 day\","
                 + "\"read-timeout\":\"1 day\",\"write-timeout\":\"1 day\",\"backoff-slot-size\":\"1 day\"}";
 
@@ -227,11 +233,11 @@ public void serDe_optional() throws Exception {
         String serializedCamelCase = "{\"apiToken\":null,\"security\":null,\"services\":{},"
                 + "\"proxyConfiguration\":null,\"connectTimeout\":null,\"readTimeout\":null,\"writeTimeout\":null,"
                 + "\"backoffSlotSize\":null,\"enableGcmCipherSuites\":null,\"enableHttp2\":null,"
-                + "\"fallbackToCommonNameVerification\":null}";
+                + "\"fallbackToCommonNameVerification\":null,\"poolReusePolicy\":null}";
         String serializedKebabCase = "{\"api-token\":null,\"security\":null,\"services\":{},"
                 + "\"proxy-configuration\":null,\"connect-timeout\":null,\"read-timeout\":null,\"write-timeout\":null,"
                 + "\"backoff-slot-size\":null,\"enable-gcm-cipher-suites\":null,\"enable-http2\":null,"
-                + "\"fallback-to-common-name-verification\":null}";
+                + "\"fallback-to-common-name-verification\":null,\"pool-reuse-policy\":null}";
 
         assertThat(ObjectMappers.newClientObjectMapper().writeValueAsString(deserialized))
                 .isEqualTo(serializedCamelCase);