java 70 lines · 8 steps

Read/write datasource routing in Spring

How Spring's AbstractRoutingDataSource picks between primary and replica databases per request using a thread-local key.

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Spring Intermediate routing threadlocal read-replica dependency-injection connection-pooling
1public enum DataSourceType {
2 PRIMARY,
3 REPLICA
4}
5 
6public final class DataSourceContextHolder {
7 
8 private static final ThreadLocal<DataSourceType> CONTEXT = new ThreadLocal<>();
9 
10 private DataSourceContextHolder() {
11 }
12 
13 public static void set(DataSourceType type) {
14 CONTEXT.set(type);
15 }
16 
17 public static DataSourceType get() {
18 return CONTEXT.get();
19 }
20 
21 public static void clear() {
22 CONTEXT.remove();
23 }
24}
25 
26public class RoutingDataSource extends AbstractRoutingDataSource {
27 
28 @Override
29 protected Object determineCurrentLookupKey() {
30 DataSourceType type = DataSourceContextHolder.get();
31 if (type != null) {
32 return type;
33 }
34 return TransactionSynchronizationManager.isCurrentTransactionReadOnly()
35 ? DataSourceType.REPLICA
36 : DataSourceType.PRIMARY;
37 }
38}
39 
40@Configuration
41public class DataSourceConfig {
42 
43 @Bean
44 @ConfigurationProperties("app.datasource.primary")
45 public DataSource primaryDataSource() {
46 return DataSourceBuilder.create().build();
47 }
48 
49 @Bean
50 @ConfigurationProperties("app.datasource.replica")
51 public DataSource replicaDataSource() {
52 return DataSourceBuilder.create().build();
53 }
54 
55 @Bean
56 @Primary
57 public DataSource routingDataSource(
58 @Qualifier("primaryDataSource") DataSource primary,
59 @Qualifier("replicaDataSource") DataSource replica) {
60 
61 RoutingDataSource routing = new RoutingDataSource();
62 Map<Object, Object> targets = new EnumMap<>(DataSourceType.class);
63 targets.put(DataSourceType.PRIMARY, primary);
64 targets.put(DataSourceType.REPLICA, replica);
65 
66 routing.setTargetDataSources(targets);
67 routing.setDefaultTargetDataSource(primary);
68 return new LazyConnectionDataSourceProxy(routing);
69 }
70}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1A ThreadLocal lets you carry routing intent through a call stack without threading it through method signatures.
  2. 2AbstractRoutingDataSource turns one logical DataSource into many by resolving a lookup key at connection time.
  3. 3Falling back to the transaction's read-only flag gives sensible replica routing even when no explicit hint is set.

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