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new a938aa3 docs: add rfc for meta-proxy (#657)
a938aa3 is described below
commit a938aa3580d8cef63149e218f6edf98b1fed46d3
Author: Wu Tao <[email protected]>
AuthorDate: Sun Jan 3 23:29:50 2021 -0600
docs: add rfc for meta-proxy (#657)
---
rfcs/2020-12-15-meta-proxy.md | 52 +++++++++++++++++++++++++++++++++++++++++++
1 file changed, 52 insertions(+)
diff --git a/rfcs/2020-12-15-meta-proxy.md b/rfcs/2020-12-15-meta-proxy.md
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+# Meta Proxy
+
+Previously, Pegasus offered every user with a list of MetaServer IP:Port
addresses. Client queries MetaServer to locate the ReplicaServer where a
specific data partition is hosted, and thereby it can read/write.
+
+But static IP is unsafe for a service, especially when we need to replace a
MetaServer node. One solution is to use domain name to access MetaServer.
+
+It's still unrealistic to manage one domain name per cluster. A better
approach is to use one domain name for multiple clusters and hide the detail of
clusters from users.
+
+One pratical design for this idea is to implement a proxy service, that could
"intelligently" routing the client queries to the matched MetaServer. This
service we call it MetaProxy.
+
+Another case of MetaProxy is the dynamic and senseless reconfiguration of the
primary cluster during duplication. While the primary cluster is crashed, the
proxy can forward client queries to the backup cluster for disaster recovery.
+
+The implementation of MetaProxy is a service that could process Pegasus's RPC
protocol. It could receive and parse the RPC request from clients. After
forwarding the query to the corresponding MetaServer, it needs to return the
response completely to the client.
+
+Every client query, ie. QueryConfig RPC, needs to specify a table name in
order to retrieve the addresses of partitions in the table.
+
+```thrift
+struct query_config_request
+{
+ 1:string app_name;
+ 2:list<i32> partition_indices;
+}
+
+struct query_config_response
+{
+ 1:base.error_code err;
+ 2:i32 app_id;
+ 3:i32 partition_count;
+ 4:bool is_stateful;
+ 5:list<partition_configuration> partitions;
+}
+```
+
+MetaProxy requires the exactly one-one mapping of table and cluster, i.e. one
table can exist in only one globally unique cluster. Thus MetaProxy can
certainly find the cluster using the table name.
+The one-one mapping should be guaranteed by the Pegasus service administrator.
+
+The cluster-to-table mapping is stored in MetaServer. To find cluster by
table, we need to store the reversed mapping in Zookeeper.
+
+The zk layout:
+
+```
+<root>/c3srv-pegasus/<table>
+
+{
+ "cluster" : "c3srv-ad",
+ "meta-addrs" : ["127.0.0.1:34601"]
+}
+```
+
+All tables under one MetaProxy deployment are unique. It's impossible for two
tables with the same name. However, we can deploy two sets of MetaProxy in two
isolated AZs, then they can both have the same table. And consequently, we can
achieve double-alive with intra-AZs.
+
+MetaProxy lazily caches each [zknode watch
event](https://zookeeper.apache.org/doc/r3.3.5/zookeeperProgrammers.html#ch_zkWatches)
for a table. If it happens to a zknode that's queried but not cached,
MetaProxy will invoke zookeeper getData API to fetch the zknode data, and
listens for its changes.
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