Sharding

docs/design/AZURE_FEDERATIONS.md

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+# Azure Federations
+
+Implementing Federations inside a new Doctrine Sharding Extension. Some extensions to the DBAL and ORM core have to be done to get this working.
+
+1. DBAL (Database Abstraction Layer)
+
+* Add support for Database Schema Operations
+    * CREATE FEDERATION
+    * CREATE TABLE ... FEDERATED ON
+    * Add support to create a multi-tenent schema from any given schema
+* Add API to pick a shard based on distribution key and atomic value
+* Add API to ask about federations, federation members and so on.
+* Add Sharding Abstraction
+    * If a shard is picked via distribution key and atomic value fire queries against this only
+    * Or query the global database.
+
+2. ORM (Object-Relational Mapper)
+
+* Federation Key has to be part of the clustered index of the table
+    * Test with a pure Multi-Tenent App with Filtering = ON (TaskList)
+    * Test with sharded app (Weather)
+
+## Implementation Details
+
+SQL Azure requires one and exactly one clustered index. It makes no difference if the primary key
+or any other key is the clustered index. Sharding requires an external ID generation (no auto-increment)
+such as GUIDs. GUIDs have negative properties with regard to clustered index performance, so that
+typically you would add a "created" timestamp for example that holds the clustered index instead
+of making the GUID a clustered index.
+
+## Example API:
+
+    @@@ php
+    <?php
+    use Doctrine\DBAL\DriverManager;
+
+    $dbParams = array(
+        'dbname' => 'tcp:dbname.database.windows.net',
+        'sharding' => array(
+            'federationName'   => 'Orders_Federation',
+            'distributionKey'  => 'CustID',
+            'distributionType' => 'integer',
+            'filteringEnabled' => false,
+        ),
+        // ...
+    );
+
+    $conn = DriverManager::getConnection($dbParams);
+    $shardManager = $conn->getShardManager();
+
+    // Example 1: query against root database
+    $sql = "SELECT * FROM Products";
+    $rows = $conn->executeQuery($sql);
+
+    // Example 2:  query against the selected shard with CustomerId = 100
+    $aCustomerID = 100;
+    $shardManager->selectShard($aCustomerID); // Using Default federationName and distributionKey
+    // Query: "USE FEDERATION Orders_Federation (CustID = $aCustomerID) WITH RESET, FILTERING OFF;"
+
+    $sql = "SELECT * FROM Customers";
+    $rows = $conn->executeQuery($sql);
+
+    // Example 3: Reset API to root database again
+    $shardManager->selectGlobal();
+
+## ID Generation
+
+With sharding all the ids have to be generated for global uniqueness. There are three strategies for this.
+
+1. Use GUIDs as described here http://blogs.msdn.com/b/cbiyikoglu/archive/2011/06/20/id-generation-in-federations-identity-sequences-and-guids-uniqueidentifier.aspx
+2. Having a central table that is accessed with a second connection to generate sequential ids
+3. Using natural keys from the domain.
+
+The second approach has the benefit of having numerical primary keys, however also a central failure location. The third strategy can seldom be used, because the domains dont allow this. Identity columns cannot be used at all.
+
+    @@@ php
+    <?php
+    use Doctrine\DBAL\DriverManager;
+    use Doctrine\DBAL\Id\TableHiLoIdGenerator;
+
+    $dbParams = array(
+        'dbname' => 'dbname.database.windows.net',
+        // ...
+    );
+    $conn = DriverManager::getConnection($dbParams);
+
+    $idGenerator = new TableHiLoIdGenerator($conn, 'id_table_name', $multiplicator = 1);
+    // only once, create this table
+    $idGenerator->createTable();
+
+    $nextId = $idGenerator->generateId('for_table_name');
+    $nextOtherId = $idGenerator->generateId('for_other_table');
+
+The connection for the table generator has to be a different one than the one used for the main app to avoid transaction clashes.

docs/design/SHARDING.md

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+# Doctrine Shards
+
+Doctrine Extension to support horizontal sharding in the Doctrine ORM.
+
+## Idea
+
+Implement sharding inside Doctrine at a level that is as unobtrusive to the developer as possible.
+
+Problems to tackle:
+
+1. Where to send INSERT statements?
+2. How to generate primary keys?
+3. How to pick shards for update, delete statements?
+4. How to pick shards for select operations?
+5. How to merge select queries that span multiple shards?
+6. How to handle/prevent multi-shard queries that cannot be merged (GROUP BY)?
+7. How to handle non-sharded data? (static metadata tables for example)
+8. How to handle multiple connections?
+9. Implementation on the DBAL or ORM level?
+
+## Roadmap
+
+Version 1: DBAL 2.3 (Multi-Tenant Apps)
+
+    1. ID Generation support (in DBAL + ORM done)
+    2. Multi-Tenant Support: Either pick a global metadata database or exactly one shard.
+    3. Fan-out queries over all shards (or a subset) by result appending
+
+Version 2: ORM related (complex):
+
+    4. ID resolving (Pick shard for a new ID)
+    5. Query resolving (Pick shards a query should send to)
+    6. Shard resolving (Pick shards an ID could be on)
+    7. Transactions
+    8. Read Only objects
+
+## Technical Requirements for Database Schemas
+
+Sharded tables require the sharding-distribution key as one of their columns. This will affect your code compared to a normalized db-schema. If you have a Blog <-> BlogPost <-> PostComments entity setup sharded by `blog_id` then even the PostComment table needs this column, even if an "unsharded", normalized DB-Schema does not need this information.
+
+## Implementation Details
+
+Assumptions:
+
+* For querying you either want to query ALL or just exactly one shard.
+* IDs for ALL sharded tables have to be unique across all shards.
+* Non-shareded data is replicated between all shards. They redundantly keep the information available. This is necessary so join queries on shards to reference data work.
+* If you retrieve an object A from a shard, then all references and collections of this object reside on the same shard.
+* The database schema on all shards is the same (or compatible)
+
+### SQL Azure Federations
+
+SQL Azure is a special case, points 1, 2, 3, 4, 7 and 8 are partly handled on the database level. This makes it a perfect test-implementation for just the subset of features in points 5-6. However there need to be a way to configure SchemaTool to generate the correct Schema on SQL Azure.
+
+* SELECT Operations: The most simple assumption is to always query all shards unless the user specifies otherwise explicitly.
+* Queries can be merged in PHP code, this obviously does not work for DISTINCT, GROUP BY and ORDER BY queries.
+
+### Generic Sharding
+
+More features are necessary to implement sharding on the PHP level, independent from database support:
+
+1. Configuration of multiple connections, one connection = one shard.
+2. Primary Key Generation mechanisms (UUID, central table, sequence emulation)
+
+## Primary Use-Cases
+
+1. Multi-Tenant Applications
+
+These are easier to support as you have some value to determine the shard id for the whole request very early on.
+Here also queries can always be limited to a single shard.
+
+2. Scale-Out by some attribute (Round-Robin?)
+
+This strategy requires access to multiple shards in a single request based on the data accessed.

docs/examples/sharding/README.md

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+# Sharding with SQLAzure Example
+
+This example demonstrates Sharding with SQL Azure Federations.
+
+## Requirements
+
+1. Windows Azure Account
+2. SQL Azure Database
+3. Composer for dependencies
+
+## Install
+
+    composer install
+
+Change "examples/sharding/bootstrap.php" to contain Database connection.
+
+## Order to execute Scripts
+
+1. create_schema.php
+2. view_federation_members.php
+3. insert_data.php
+4. split_federation.php
+5. insert_data_after_split.php
+6. query_filtering_off.php
+7. query_filtering_on.php
+

docs/examples/sharding/bootstrap.php

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+<?php
+// bootstrap.php
+use Doctrine\DBAL\DriverManager;
+use Doctrine\Shards\DBAL\SQLAzure\SQLAzureShardManager;
+
+require_once "vendor/autoload.php";
+
+$config = array(
+    'dbname'   => 'SalesDB',
+    'host'     => 'tcp:dbname.windows.net',
+    'user'     => 'user@dbname',
+    'password' => 'XXX',
+    'sharding' => array(
+        'federationName'   => 'Orders_Federation',
+        'distributionKey'  => 'CustId',
+        'distributionType' => 'integer',
+    )
+);
+
+if ($config['host'] == "tcp:dbname.windows.net") {
+    die("You have to change the configuration to your Azure account.\n");
+}
+
+$conn = DriverManager::getConnection($config);
+$shardManager = new SQLAzureShardManager($conn);
+

docs/examples/sharding/composer.json

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+{
+    "require": {
+        "doctrine/dbal": "*",
+        "doctrine/shards": "0.3"
+    }
+}

docs/examples/sharding/create_schema.php

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+<?php
+// create_schema.php
+use Doctrine\DBAL\Schema\Schema;
+use Doctrine\Shards\DBAL\SQLAzure\SQLAzureSchemaSynchronizer;
+
+require_once 'bootstrap.php';
+
+$schema = new Schema();
+
+$products = $schema->createTable('Products');
+$products->addColumn('ProductID', 'integer');
+$products->addColumn('SupplierID', 'integer');
+$products->addColumn('ProductName', 'string');
+$products->addColumn('Price', 'decimal', array('scale' => 2, 'precision' => 12));
+$products->setPrimaryKey(array('ProductID'));
+$products->addOption('azure.federated', true);
+
+$customers = $schema->createTable('Customers');
+$customers->addColumn('CustomerID', 'integer');
+$customers->addColumn('CompanyName', 'string');
+$customers->addColumn('FirstName', 'string');
+$customers->addColumn('LastName', 'string');
+$customers->setPrimaryKey(array('CustomerID'));
+$customers->addOption('azure.federated', true);
+$customers->addOption('azure.federatedOnColumnName', 'CustomerID');
+
+$orders = $schema->createTable('Orders');
+$orders->addColumn('CustomerID', 'integer');
+$orders->addColumn('OrderID', 'integer');
+$orders->addColumn('OrderDate', 'datetime');
+$orders->setPrimaryKey(array('CustomerID', 'OrderID'));
+$orders->addOption('azure.federated', true);
+$orders->addOption('azure.federatedOnColumnName', 'CustomerID');
+
+$orderItems = $schema->createTable('OrderItems');
+$orderItems->addColumn('CustomerID', 'integer');
+$orderItems->addColumn('OrderID', 'integer');
+$orderItems->addColumn('ProductID', 'integer');
+$orderItems->addColumn('Quantity', 'integer');
+$orderItems->setPrimaryKey(array('CustomerID', 'OrderID', 'ProductID'));
+$orderItems->addOption('azure.federated', true);
+$orderItems->addOption('azure.federatedOnColumnName', 'CustomerID');
+
+// Create the Schema + Federation:
+$synchronizer = new SQLAzureSchemaSynchronizer($conn, $shardManager);
+
+// Or jut look at the SQL:
+echo implode("\n", $synchronizer->getCreateSchema($schema));
+
+$synchronizer->createSchema($schema);
+