Another example of a sharding function (#41)

* Another example of a sharding function

* tests

pgcat.toml

@@ -96,3 +96,13 @@ query_parser_enabled = false
 # load balancing of read queries. Otherwise, the primary will only be used for write
 # queries. The primary can always be explicitely selected with our custom protocol.
 primary_reads_enabled = true
+
+# So what if you wanted to implement a different hashing function,
+# or you've already built one and you want this pooler to use it?
+#
+# Current options:
+#
+# pg_bigint_hash: PARTITION BY HASH (Postgres hashing function)
+# sha1: A hashing function based on SHA1
+#
+sharding_function = "pg_bigint_hash"

src/config.rs

@@ -118,6 +118,7 @@ pub struct QueryRouter {
     pub default_role: String,
     pub query_parser_enabled: bool,
     pub primary_reads_enabled: bool,
+    pub sharding_function: String,
 }
 
 impl Default for QueryRouter {
@@ -126,6 +127,7 @@ impl Default for QueryRouter {
             default_role: String::from("any"),
             query_parser_enabled: false,
             primary_reads_enabled: true,
+            sharding_function: "pg_bigint_hash".to_string(),
         }
     }
 }
@@ -159,6 +161,8 @@ impl Config {
             self.general.healthcheck_timeout
         );
         info!("Connection timeout: {}ms", self.general.connect_timeout);
+        info!("Sharding function: {}", self.query_router.sharding_function);
+        info!("Number of shards: {}", self.shards.len());
     }
 }
 
@@ -193,6 +197,18 @@ pub async fn parse(path: &str) -> Result<(), Error> {
         }
     };
 
+    match config.query_router.sharding_function.as_ref() {
+        "pg_bigint_hash" => (),
+        "sha1" => (),
+        _ => {
+            error!(
+                "Supported sharding functions are: 'pg_bigint_hash', 'sha1', got: '{}'",
+                config.query_router.sharding_function
+            );
+            return Err(Error::BadConfig);
+        }
+    };
+
     // Quick config sanity check.
     for shard in &config.shards {
         // We use addresses as unique identifiers,

src/query_router.rs

@@ -1,5 +1,5 @@
 use crate::config::{get_config, Role};
-use crate::sharding::Sharder;
+use crate::sharding::{Sharder, ShardingFunction};
 /// Route queries automatically based on explicitely requested
 /// or implied query characteristics.
 use bytes::{Buf, BytesMut};
@@ -48,6 +48,9 @@ pub struct QueryRouter {
 
     // Should we try to parse queries?
     query_parser_enabled: bool,
+
+    // Which sharding function are we using?
+    sharding_function: ShardingFunction,
 }
 
 impl QueryRouter {
@@ -76,6 +79,12 @@ impl QueryRouter {
             _ => unreachable!(),
         };
 
+        let sharding_function = match config.query_router.sharding_function.as_ref() {
+            "pg_bigint_hash" => ShardingFunction::PgBigintHash,
+            "sha1" => ShardingFunction::Sha1,
+            _ => unreachable!(),
+        };
+
         QueryRouter {
             default_server_role: default_server_role,
             shards: config.shards.len(),
@@ -84,6 +93,7 @@ impl QueryRouter {
             active_shard: None,
             primary_reads_enabled: config.query_router.primary_reads_enabled,
             query_parser_enabled: config.query_router.query_parser_enabled,
+            sharding_function,
         }
     }
 
@@ -139,8 +149,8 @@ impl QueryRouter {
 
         match command {
             Command::SetShardingKey => {
-                let sharder = Sharder::new(self.shards);
-                let shard = sharder.pg_bigint_hash(value.parse::<i64>().unwrap());
+                let sharder = Sharder::new(self.shards, self.sharding_function);
+                let shard = sharder.shard(value.parse::<i64>().unwrap());
                 self.active_shard = Some(shard);
                 value = shard.to_string();
             }

src/sharding.rs

@@ -1,26 +1,62 @@
+use sha1::{Digest, Sha1};
+
 // https://github.com/postgres/postgres/blob/27b77ecf9f4d5be211900eda54d8155ada50d696/src/include/catalog/partition.h#L20
 const PARTITION_HASH_SEED: u64 = 0x7A5B22367996DCFD;
 
+#[derive(Debug, PartialEq, Copy, Clone)]
+pub enum ShardingFunction {
+    PgBigintHash,
+    Sha1,
+}
+
 pub struct Sharder {
     shards: usize,
+    sharding_function: ShardingFunction,
 }
 
 impl Sharder {
-    pub fn new(shards: usize) -> Sharder {
-        Sharder { shards: shards }
+    pub fn new(shards: usize, sharding_function: ShardingFunction) -> Sharder {
+        Sharder {
+            shards,
+            sharding_function,
+        }
+    }
+
+    pub fn shard(&self, key: i64) -> usize {
+        match self.sharding_function {
+            ShardingFunction::PgBigintHash => self.pg_bigint_hash(key),
+            ShardingFunction::Sha1 => self.sha1(key),
+        }
     }
 
     /// Hash function used by Postgres to determine which partition
     /// to put the row in when using HASH(column) partitioning.
     /// Source: https://github.com/postgres/postgres/blob/27b77ecf9f4d5be211900eda54d8155ada50d696/src/common/hashfn.c#L631
     /// Supports only 1 bigint at the moment, but we can add more later.
-    pub fn pg_bigint_hash(&self, key: i64) -> usize {
+    fn pg_bigint_hash(&self, key: i64) -> usize {
         let mut lohalf = key as u32;
         let hihalf = (key >> 32) as u32;
         lohalf ^= if key >= 0 { hihalf } else { !hihalf };
         Self::combine(0, Self::pg_u32_hash(lohalf)) as usize % self.shards
     }
 
+    /// Example of a hashing function based on SHA1.
+    fn sha1(&self, key: i64) -> usize {
+        let mut hasher = Sha1::new();
+
+        hasher.update(&key.to_string().as_bytes());
+
+        let result = hasher.finalize();
+
+        // Convert the SHA1 hash into hex so we can parse it as a large integer.
+        let hex = format!("{:x}", result);
+
+        // Parse the last 8 bytes as an integer (8 bytes = bigint).
+        let key = i64::from_str_radix(&hex[hex.len() - 8..], 16).unwrap() as usize;
+
+        key % self.shards
+    }
+
     #[inline]
     fn rot(x: u32, k: u32) -> u32 {
         (x << k) | (x >> (32 - k))
@@ -109,36 +145,51 @@ mod test {
     // confirming that we implemented Postgres BIGINT hashing correctly.
     #[test]
     fn test_pg_bigint_hash() {
-        let sharder = Sharder::new(5);
+        let sharder = Sharder::new(5, ShardingFunction::PgBigintHash);
 
         let shard_0 = vec![1, 4, 5, 14, 19, 39, 40, 46, 47, 53];
 
         for v in shard_0 {
-            assert_eq!(sharder.pg_bigint_hash(v), 0);
+            assert_eq!(sharder.shard(v), 0);
         }
 
         let shard_1 = vec![2, 3, 11, 17, 21, 23, 30, 49, 51, 54];
 
         for v in shard_1 {
-            assert_eq!(sharder.pg_bigint_hash(v), 1);
+            assert_eq!(sharder.shard(v), 1);
         }
 
         let shard_2 = vec![6, 7, 15, 16, 18, 20, 25, 28, 34, 35];
 
         for v in shard_2 {
-            assert_eq!(sharder.pg_bigint_hash(v), 2);
+            assert_eq!(sharder.shard(v), 2);
         }
 
         let shard_3 = vec![8, 12, 13, 22, 29, 31, 33, 36, 41, 43];
 
         for v in shard_3 {
-            assert_eq!(sharder.pg_bigint_hash(v), 3);
+            assert_eq!(sharder.shard(v), 3);
         }
 
         let shard_4 = vec![9, 10, 24, 26, 27, 32, 37, 38, 42, 45];
 
         for v in shard_4 {
-            assert_eq!(sharder.pg_bigint_hash(v), 4);
+            assert_eq!(sharder.shard(v), 4);
+        }
+    }
+
+    #[test]
+    fn test_sha1_hash() {
+        let sharder = Sharder::new(12, ShardingFunction::Sha1);
+        let ids = vec![
+            0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19,
+        ];
+        let shards = vec![
+            4, 7, 8, 3, 6, 0, 0, 10, 3, 11, 1, 7, 4, 4, 11, 2, 5, 0, 8, 3,
+        ];
+
+        for (i, id) in ids.iter().enumerate() {
+            assert_eq!(sharder.shard(*id), shards[i]);
         }
     }
 }