In the first part of the three-part blog series, we reviewed the automated data distribution and centralized schema management capabilities of Oracle Sharding. In part 2, I will cover the automated deployment of a sharded database (SDB) and data-dependent routing against an SDB.
Automated creation and replication of shards
In the Oracle Database 126.96.36.199 release, Oracle Sharding supports three automatically configured replication options: Data Guard, Active Data Guard, or Oracle GoldenGate (Oracle GoldenGate 12.3 supports Oracle Sharding).
Figure 2. Shard-level replication with Active Data Guard
Figure 3. chunkset-level replication with Oracle GoldenGate
Sharding supports two SDB deployment methods.
The first method is with the “CREATE SHARD” GDSCTL command. With this method, the shards and their respective listeners are automatically created. Once the primary shards are created, the corresponding standby shards are automatically built using the RMAN ‘duplicate’ command. After which, the Data Guard Broker configuration with Fast-Start Failover (FSFO) is automatically enabled. The FSFO observers are automatically started on the regional shard director.
The second method is with the “ADD SHARD” GDSCTL command. Many organizations have their own database creation standards and they may opt to deploy the SDB using their own pre-created databases (shards). The ADD SHARD based deployment method supports this requirement by simply adding the shards, which are pre-built by the user.
These two deployment methods support both the initial and incremental deployments.
Oracle Sharding supports Direct and Proxy routing of database requests to shards.
Key enhancements have been made to Oracle connection pools and drivers to support Sharding. Starting from 12.2, JDBC/UCP, OCI and Oracle Data Provider for .NET (ODP.NET) recognize the sharding keys as part of the connection check out. Apache Tomcat, JBoss, IBM WebSphere and Oracle WebLogic can use UCP support for sharding. PHP, Python, Perl, and Node.js can use OCI support.
Sharding Key is used for routing the database connection requests at a user session level during connection checkout. Based on this information, connection is established to the relevant shard which contains the data pertinent to the given sharding_key. Once the session is made to a shard, all SQL queries and DMLs are supported, executed in the scope of the given shard and require no modification.
Upon the first connection to a given shard, the sharding key range mapping is collected from the shards to dynamically build the shard topology cache. This routing map is cached in the client. This allows subsequent requests for sharding keys within the cached range to be routed directly to the shard, bypassing the shard director. Such data-dependent routing of database requests eliminates an extra network-hop – thereby decreasing the latency for high volume OLTP applications.
When a connection request is made with a sharding key, connection pool looks up the corresponding shards on which this particular sharding key exists (from its topology cache). If a matching connection is available in the pool then the pool returns a connection to one of these shards by applying its internal connection selection algorithm. If a connection is not available, then forwarding the request with the sharding key (by the connection pool) to the shard director creates a new connection.
As illustrated in Figure 4, DB connection request for a given sharding key that is in any of the cached topology map, goes directly to the shard (i.e., bypassing the shard director).
Figure 4. Logical flow of direct routing – Connection pool as a Shard Director
Note: Super_sharding_key is needed only in the case of composite sharding.
The routing map automatically refreshes when a shard becomes unavailable or changes occur to the sharding topology. This is enabled by the Fast Application Notification (FAN) published by Shard Director via Oracle Notification Server (ONS).
Proxy Routing for Multi-Shard Queries:
Proxy routing is an ancillary usage pattern targeted for developer convenience. This requires connection be established to the coordinator. In Oracle Database 188.8.131.52, the shard catalog database assumes the role of the coordinator database. Once the session is made to the coordinator, SQL queries and DMLs are executed and require no modification. Proxy routing is suitable for the following scenarios:
» When the application cannot pass the sharding key during connect
» When the application needs to access data from multiple shards in the same query. For example, a query to aggregate data across all shards.
Figure 5. Logical flow of proxy routing
As illustrated in Figure 5, routing via the coordinator allows users to submit SQL statements without a sharding key value passed during connect. The Coordinator’s SQL compiler analyzes and rewrites the query into query fragments that are sent and executed by the participating shards. The queries are rewritten so that most of the query processing is done on the participating shards and then aggregated by the coordinator.
Applications should separate their workloads for direct vs. proxy routing. Separate connection pools must be created for these workloads.
In the second part of the three-part blog series , we have looked at the automated creation of shards and the replication setup. We also understood how routing happens against a sharded database. In the last part, we will go over the lifecycle management aspects of a sharded database.