Import Distributed Filer docs

Distributed-Filer.md

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+The default weed filer is in standalone mode, storing file metadata on disk.
+It is quite efficient to go through deep directory path and can handle
+millions of files.
+
+However, no SPOF is a must-have requirement for many projects.
+
+Luckily, SeaweedFS is so flexible that we can use a completely different way
+to manage file metadata.
+
+This distributed filer uses Redis or Cassandra to store the metadata.
+
+## Redis Setup
+
+No setup required.
+
+## Cassandra Setup
+
+Here is the CQL to create the table.CassandraStore.
+Optionally you can adjust the keyspace name and replication settings.
+For production, you would want to set replication_factor to 3
+if there are at least 3 Cassandra servers.
+
+```cql
+create keyspace seaweed WITH replication = {
+  'class':'SimpleStrategy',
+  'replication_factor':1
+};
+
+use seaweed;
+
+CREATE TABLE seaweed_files (
+   path varchar,
+   fids list<varchar>,
+   PRIMARY KEY (path)
+);
+```
+
+## Sample usage
+
+To start a weed filer in distributed mode with Redis:
+
+```bash
+# assuming you already started weed master and weed volume
+weed filer -redis.server=localhost:6379
+```
+
+To start a weed filer in distributed mode with Cassandra:
+
+```bash
+# assuming you already started weed master and weed volume
+weed filer -cassandra.server=localhost
+```
+
+Now you can add/delete files
+
+```bash
+# POST a file and read it back
+curl -F "filename=@README.md" "http://localhost:8888/path/to/sources/"
+curl "http://localhost:8888/path/to/sources/README.md"
+# POST a file with a new name and read it back
+curl -F "filename=@Makefile" "http://localhost:8888/path/to/sources/new_name"
+curl "http://localhost:8888/path/to/sources/new_name"
+```
+
+## Limitation
+
+List sub folders and files are not supported because Redis or Cassandra
+does not support prefix search.
+
+## Flat Namespace Design
+
+Instead of using both directory and file metadata, this implementation uses
+a flat namespace.
+
+If storing each directory metadata separately, there would be multiple
+network round trips to fetch directory information for deep directories,
+impeding system performance.
+
+A flat namespace would take more space because the parent directories are
+repeatedly stored. But disk space is a lesser concern especially for
+distributed systems.
+
+So either Redis or Cassandra is a simple file_full_path ~ file_id mapping.
+(Actually Cassandra is a file_full_path ~ list_of_file_ids mapping
+with the hope to support easy file appending for streaming files.)
+
+## Complexity
+
+For one file retrieval, the full_filename=>file_id lookup will be O(logN)
+using Redis or Cassandra. But very likely the one additional network hop would
+take longer than the actual lookup.
+
+## Use Cases
+
+Clients can assess one "weed filer" via HTTP, create files via HTTP POST,
+read files via HTTP POST directly.
+
+## Future
+
+SeaweedFS can support other distributed databases. It will be better
+if that database can support prefix search, in order to list files
+under a directory.
+
+## Helps Wanted
+
+Please implement your preferred metadata store!
+
+Just follow the cassandra_store/cassandra_store.go file and send me a pull
+request. I will handle the rest.