Schema design best practices

The distributed architecture of Spanner lets you design your schema to avoid hotspots - situations where too many requests are sent to the same server which saturates the resources of the server and potentially causes high latencies.

This page describes best practices for designing your schemas to avoid creating hotspots. One way to avoid hotspots is to adjust the schema design to allow Spanner to split and distribute the data across multiple servers. Distributing data across servers helps your Spanner database operate efficiently, particularly when performing bulk data insertions.

Choose a primary key to prevent hotspots

As mentioned in Schema and data model, you should be careful when choosing a primary key in the schema design to not accidentally create hotspots in your database. One cause of hotspots is having a column whose value monotonically changes as the first key part, because this results in all inserts occurring at the end of your key space. This pattern is undesirable because Spanner uses key ranges to divide data among servers, which means all your inserts are directed at a single server that ends up doing all the work.

For example, suppose you want to maintain a last access timestamp column on rows of the UserAccessLog table. The following table definition uses a timestamp-based primary key as the first key part. We don't recommend this if the table sees a high rate of insertion:

CREATE TABLE UserAccessLogs (
  LastAccess TIMESTAMP NOT NULL,
  UserId STRING(1024),
  ...
) PRIMARY KEY (LastAccess, UserId);

CREATE TABLE useraccesslog (
  lastaccess timestamptz NOT NULL,
  userid text,
  ...
PRIMARY KEY (lastaccess, userid)
);

The problem here is that rows are written to this table in order of last access timestamp, and because last access timestamps are always increasing, they're always written to the end of the table. The hotspot is created because a single Spanner server receives all of the writes, which overloads that one server.

The following diagram illustrates this pitfall:

The previous UserAccessLog table includes five example rows of data, which represent five different users taking some sort of user action about a millisecond apart from each other. The diagram also annotates the order in which Spanner inserts the rows (the labeled arrows indicate the order of writes for each row). Because inserts are ordered by timestamp, and the timestamp value is always increasing, Spanner always adds the inserts to the end of the table and directs them at the same split. (As discussed in Schema and data model, a split is a set of rows from one or more related tables that Spanner stores in order of row key.)

This is problematic because Spanner assigns work to different servers in units of splits, so the server assigned to this particular split ends up handling all the insert requests. As the frequency of user access events increases, the frequency of insert requests to the corresponding server also increases. The server then becomes prone to becoming a hotspot, and looks like the red border and background shown in the previous image. In this simplified illustration, each server handles at most one split but Spanner can assign each server more than one split.

When Spanner appends more rows to the table, the split grows, and when it reaches approximately 8 GB, Spanner creates another split, as described in Load-based splitting. Spanner appends subsequent new rows to this new split, and the server assigned to the split becomes the new potential hotspot.

When hotspots occur, you might observe that your inserts are slow and other work on the same server might slow down. Changing the order of the LastAccess column to ascending order doesn't solve this problem because then all the writes are inserted at the top of the table instead, which still sends all the inserts to a single server.

Schema design best practice #1: Do not choose a column whose value monotonically increases or decreases as the first key part for a high write rate table.

Use a Universally Unique Identifier (UUID)

You can use a Universally Unique Identifier (UUID) as defined by RFC 4122 as the primary key. We recommend using UUID Version 4, because it uses random values in the bit sequence. We don't recommend Version 1 UUIDs because they store the timestamp in the high order bits.

There are several ways to store the UUID as the primary key:

• In a STRING(36) column.
• In a pair of INT64 columns.
• In a BYTES(16) column.

For a STRING(36) column, you can use the Spanner GENERATE_UUID() function (GoogleSQL or PostgreSQL) as the column default value to have Spanner automatically generate UUID values.

For example, for the following table:

CREATE TABLE UserAccessLogs (
  LogEntryId STRING(36) NOT NULL,
  LastAccess TIMESTAMP NOT NULL,
  UserId STRING(1024),
  ...
) PRIMARY KEY (LogEntryId, LastAccess, UserId);

CREATE TABLE useraccesslog (
  logentryid VARCHAR(36) NOT NULL,
  lastaccess timestamptz NOT NULL,
  userid text,
  ...
PRIMARY KEY (lastaccess, userid)
);

You could insert GENERATE_UUID() to generate the LogEntryId values. GENERATE_UUID() produces a STRING value, so the LogEntryId column must use the STRING type for GoogleSQL, or the text type for PostgreSQL.

INSERT INTO
  UserAccessLog (LogEntryId, LastAccess, UserId)
VALUES
  (GENERATE_UUID(), '2016-01-25 10:10:10.555555-05:00', 'TomSmith');

INSERT INTO
  useraccesslog (logentryid, lastaccess, userid)
VALUES
  (spanner.generate_uuid(),'2016-01-25 10:10:10.555555-05:00', 'TomSmith');

There are a few disadvantages to using a UUID:

• They are slightly large, using 16 bytes or more. Other options for primary keys don't use this much storage.
• They don't carry information about the record. For example, a primary key of SingerId and AlbumId has an inherent meaning, while a UUID doesn't.
• You lose locality between related records, which is why using a UUID eliminates hotspots.

Bit-reverse sequential values

You should ensure that numerical (INT64 in GoogleSQL or bigint in PostgreSQL) primary keys aren't sequentially increasing or decreasing. Sequential primary keys can cause hotspots at scale. One way to avoid this problem is to bit-reverse the sequential values, making sure to distribute primary key values evenly across the key space.

Spanner supports bit-reversed sequence, which generates unique integer bit-reversed values. You can use a sequence in the first (or only) component in a primary key to avoid hotspot issues. For more information, see Bit-reversed sequence.

Swap the order of keys

One way to spread writes over the key space more uniformly is to swap the order of the keys so that the column that contains the monotonic value is not the first key part:

CREATE TABLE UserAccessLog (
UserId     INT64 NOT NULL,
LastAccess TIMESTAMP NOT NULL,
...
) PRIMARY KEY (UserId, LastAccess);

CREATE TABLE useraccesslog (
userid bigint NOT NULL,
lastaccess TIMESTAMPTZ NOT NULL,
...
PRIMARY KEY (UserId, LastAccess)
);

In this modified schema, inserts are now first ordered by UserId, rather than by chronological last access timestamp. This schema spreads writes among different splits because it's unlikely that a single user produces thousands of events per second.

The following image shows the five rows from the UserAccessLog table that Spanner orders with UserId instead of access timestamp:

Here Spanner chunks the UserAccessLog data into three splits, with each split containing approximately a thousand rows of ordered UserId values. This is a reasonable estimate of how the user data could be split, assuming each row contains about 1MB of user data and given a maximum split size of approximately 8 GB. Even though the user events occurred about a millisecond apart, each event was raised by a different user, so the order of inserts is much less likely to create a hotspot compared with using the timestamp for ordering.

See also the related best practice for ordering timestamp-based keys.

Hash the unique key and spread the writes across logical shards

Another common technique for spreading the load across multiple servers is to create a column that contains the hash of the actual unique key, then use the hash column (or the hash column and the unique key columns together) as the primary key. This pattern helps avoid hotspots, because new rows are spread more evenly across the key space.

You can use the hash value to create logical shards, or partitions, in your database. In a physically sharded database, the rows are spread across several database servers. In a logically sharded database, the data in the table define the shards. For example, to spread writes to the UserAccessLog table across N logical shards, you could prepend a ShardId key column to the table:

CREATE TABLE UserAccessLog (
ShardId     INT64 NOT NULL,
LastAccess  TIMESTAMP NOT NULL,
UserId      INT64 NOT NULL,
...
) PRIMARY KEY (ShardId, LastAccess, UserId);

CREATE TABLE useraccesslog (
shardid bigint NOT NULL,
lastaccess TIMESTAMPTZ NOT NULL,
userid bigint NOT NULL,
...
PRIMARY KEY (shardid, lastaccess, userid)
);

To compute the ShardId, hash a combination of the primary key columns and then calculate modulo N of the hash. For example:

ShardId = hash(LastAccess and UserId) % N

Your choice of hash function and combination of columns determines how the rows are spread across the key space. Spanner will then create splits across the rows to optimize performance.

The following diagram illustrates how using a hash to create three logical shards can spread write throughput more evenly across servers:

Here the UserAccessLog table is ordered by ShardId, which is calculated as a hash function of key columns. The five UserAccessLog rows are chunked into three logical shards, each of which is coincidentally in a different split. The inserts are spread evenly among the splits, which balances write throughput to the three servers that handle the splits.

Spanner also lets you create a hash function in a generated column.

To do this in GoogleSQL, use the FARM_FINGERPRINT function during write time, as shown in the following example:

CREATE TABLE UserAccessLog (
ShardId INT64 NOT NULL
AS (MOD(FARM_FINGERPRINT(CAST(LastAccess AS STRING)), 2048)) STORED,
LastAccess TIMESTAMP NOT NULL,
UserId    INT64 NOT NULL,
) PRIMARY KEY (ShardId, LastAccess, UserId);

Your choice of hash function determines how well your insertions are spread across the key range. You don't need a cryptographic hash, although a cryptographic hash might be a good choice. When picking a hash function, you need to consider the following factors:

• Hotspot avoidance. A function that results in more hash values tends to reduce hotspots.
• Read efficiency. Reads across all hash values are faster if there are fewer hash values to scan.
• Node count.

Use descending order for timestamp-based keys

If you have a table for your history that uses the timestamp as a key, consider using descending order for the key column if any of the following apply:

• If you want to read the most recent history, you're using an interleaved table for the history, and you're reading the parent row. In this case, with a DESC timestamp column, the latest history entries are stored adjacent to the parent row. Otherwise, reading the parent row and its recent history will require a seek in the middle to skip over the older history.
• If you're reading sequential entries in reverse chronological order, and you don't know exactly how far back you're going. For example, you might use a SQL query with a LIMIT to get the most recent N events, or you might plan to cancel the read after you've read a certain number of rows. In these cases, you want to start with the most recent entries and read sequentially older entries until your condition has been met, which Spanner does more efficiently for timestamp keys that Spanner stores in descending order.

Add the DESC keyword to make the timestamp key descending. For example:

CREATE TABLE UserAccessLog (
UserId     INT64 NOT NULL,
LastAccess TIMESTAMP NOT NULL,
...
) PRIMARY KEY (UserId, LastAccess DESC);

Schema design best practice #2: Descending order or ascending order depends on the user queries, for example, top being the newest, or top being the oldest.

Use an interleaved index on a column whose value monotonically increases or decreases

Similar to the previous primary key example that you should avoid, it's also a bad idea to create non-interleaved indexes on columns whose values are monotonically increasing or decreasing, even if they aren't primary key columns.

For example, suppose you define the following table, in which LastAccess is a non-primary-key column:

CREATE TABLE Users (
UserId     INT64 NOT NULL,
LastAccess TIMESTAMP,
...
) PRIMARY KEY (UserId);

CREATE TABLE Users (
userid     bigint NOT NULL,
lastaccess TIMESTAMPTZ,
...
PRIMARY KEY (userid)
);

It might seem convenient to define an index on the LastAccess column for quickly querying the database for user accesses "since time X", like this:

CREATE NULL_FILTERED INDEX UsersByLastAccess ON Users(LastAccess);

CREATE INDEX usersbylastaccess ON users(lastaccess)
WHERE lastaccess IS NOT NULL;

However, this results in the same pitfall as described in the previous best practice, because Spanner implements indexes as tables under the hood, and the resulting index table uses a column whose value monotonically increases as its first key part.

It's okay to create an interleaved index like this though, because rows of interleaved indexes are interleaved in corresponding parent rows, and it's unlikely for a single parent row to produce thousands of events per second.

Schema design best practice #3: Do not create a non-interleaved index on a high write rate column whose value monotonically increases or decreases. Instead of using interleaved indexes, use techniques like those you would use for the base table primary key design when designing index columns—for example, add `shardId`.

What's next

• Look through examples of schema designs.
• Learn about bulk loading data.