Add Sharded Bucket middleware

Adds the sharded bucket middleware, which allows for splitting objects
across multiple backend buckets for a given virtual bucket. The
middleware should be configured as:
s3proxy.sharded-blobstore.<bucket name>.shards=<number of shards>
s3proxy.sharded-blobstore.<bucket name>.prefix=<prefix>.

All shards are named <prefix>-<index>, where index is an
integer from 0 to <number of shards> - 1. If the <prefix> is not
supplied, the <bucket name> is used as the prefix.

Listing the virtual bucket and multipart uploads are not supported. When
listing all containers, the shards are elided from the result.

Fixes gaul#325
Fixes gaul#351

README.md

@@ -90,6 +90,7 @@ S3Proxy can modify its behavior based on middlewares:
 * [eventual consistency modeling](https://github.com/gaul/s3proxy/wiki/Middleware---eventual-consistency)
 * [large object mocking](https://github.com/gaul/s3proxy/wiki/Middleware-large-object-mocking)
 * [read-only](https://github.com/gaul/s3proxy/wiki/Middleware-read-only)
+* [sharded backend containers](https://github.com/gaul/s3proxy/wiki/Middleware-sharded-backend)
 
 ## Limitations
 

src/main/java/org/gaul/s3proxy/Main.java

@@ -224,6 +224,16 @@ private static BlobStore parseMiddlewareProperties(BlobStore blobStore,
             blobStore = ReadOnlyBlobStore.newReadOnlyBlobStore(blobStore);
         }
 
+        ImmutableMap<String, Integer> shards =
+                ShardedBlobStore.parseBucketShards(properties);
+        ImmutableMap<String, String> prefixes =
+                ShardedBlobStore.parsePrefixes(properties);
+        if (!shards.isEmpty()) {
+            System.err.println("Using sharded buckets backend");
+            blobStore = ShardedBlobStore.newShardedBlobStore(blobStore,
+                    shards, prefixes);
+        }
+
         return blobStore;
     }
 

src/main/java/org/gaul/s3proxy/S3ProxyConstants.java

@@ -84,6 +84,9 @@ public final class S3ProxyConstants {
     /** Prevent mutations. */
     public static final String PROPERTY_READ_ONLY_BLOBSTORE =
             "s3proxy.read-only-blobstore";
+    /** Shard objects across a specified number of buckets. */
+    public static final String PROPERTY_SHARDED_BLOBSTORE =
+            "s3proxy.sharded-blobstore";
 
     /** Maximum time skew allowed in signed requests. */
     public static final String PROPERTY_MAXIMUM_TIME_SKEW =