1.0 Introduction
In the last 20 years, software systems moved to the internet and handled large volumes of data and millions of requests. Most people interact with these systems using a browser or a mobile device. At the back end, is not one powerful computer but generally a network of commodity computers. Both the processing and storage of data is spread across multiple computers. In this blog we discuss how large datasets can be stored using multiple commodity computers.
Partitioning is the process of breaking up a large dataset in parts so that each part can fit easily on the disk on one one node and be efficiently managed by each node. For very large data sets that cannot fit on 1 machine, data needs to be broken up into parts ( partition or shard). Each partition is stored on a different machine. This is just natural horizontal scaling. But most important is that, when it time to read the partitioned data, we need to be able to find (efficiently) which partition and node has the data we want to read.
Storage space is not the only benefit of partitioning. You are also spreading the compute required to read, write and process the data.
Partitioning is generally combined with replication to make the partitions highly available. But we do not discuss replication here. That is a topic for another blog.
2.0 Types of partitioning
There are 2 types of data that need to be considered: unstructured and structured.
Most discussions on partitioning discuss partitioning of data in databases ( structured data ) but not unstructured data which is outside databases in plain files. This blog discusses both unstructured data and structured data.
2.1 Structured data
The problem is more interesting for databases because it is not enough to break up the dataset into smaller parts. During reads you need to be able to find the data. And you need to do it fast. When the database receives a query - "Give me records for Customer X", How does it know which node hosts the data ? Does the database have to send the request to all the nodes ? That would be quite inefficient.
The goal is thus to partition data and query it efficiently. Another goal is to ensure that distribution of data between partitions is even. You do not want a situation where partition 1 has 70% of the data and the other 5 partitions has the remaining 30%. This will overload partition 1 ( known as a hot spot) and you lose the benefits of partitioning.
2 strategies are commonly used for database partitioning.
2.1.1 Range based partitioning
Database records are generally stored sorted based on the primary key.
Initially there is one partition with zero records.
As clients write to the database, the size of the partition increases. When it reaches a certain size say 10MB or 64MB, it is split into two partitions.
Each partition may be assigned to a different node.
This process is repeated as more data is added and partitions grow. If data is deleted and partitions shrink, then small partitions can be merged
To efficiently query data, the database needs to do some book keeping
-- which key range is in which partition
-- which partition is at which node
Starting with 1 partition and 1 node is not efficient for obvious reasons and databases typically start with configured number of partitions or a number of partitions proportional to the number of nodes.
To balance the load on nodes, partitions may need to be moved between nodes.
This is the strategy used by HBase, CockroachDb, MongoDb.
2.1.2 Hash based partitioning
The hash value calculated from the key is used to determine the location where the record can be stored.
The wrong way to determine the node is by using hash mod n, where n is the number of nodes. The problem with this approach is that when nodes are added or removed from the cluster, a very high percentage of the keys need to be removed.
A better approach is to start with a fixed number of partitions , way more that the number of the nodes the cluster will ever have , say for example 1000 or 10000. Partitions are logical. Hash ranges are assigned to partitions. Partitions are assigned to nodes either using numPartitions mod numNodes or other algorithms. This is shown in the top half of Figure 4. The bottom half of figure 4 visualizes the same as a ring as is done in many articles that to refers to this as consistent hashing. Think of partitions being placed on the ring. Each partition owns the key space from the position of the previous position to its position. The cluster needs to maintain a mapping of partitions to nodes. When a new node is added, the cluster can take a few partitions from existing nodes and assign to the new nodes. When a node is removed, the cluster assigns its partitions to other nodes. Looking up a key is a extra level of indirection. The hash of key maps to a partition. The partition node map tell you which node has the partition that has the key. Many studies has shown that this lead to less movement of keys between nodes as the cluster changes.
In popular press, this has the poorly understood name "consistent hashing". It is just hast hash based partitioning. Nothing consistent and nothing to do with consistency.
2.1.3 Secondary Indexes
So far we have been talking about partitioning by the primary key, also know as the primary index.
To speed up retrieval of records, databases also have secondary indexes which can be very large and might need to be partitioned.
One approach can be to keep the secondary indexes local to the node on which the primary index partition is. The advantage of this approach is that since all related rows are on the same node, inserts/updates/deletes are are all local. But queries on the secondary index requires sending queries to all nodes and aggregating the responses.
Another approach is to create a global secondary index and partition it as an independent entity. However since the secondary index partitions might be on a different partition from the primary partition, CUD operations are more expensive. Transactions might be distributed. However range queries on secondary indexes are more efficient since closer records (by sort) are on the same partition.
2.2 Unstructured data
Unstructured data refer to ordinary files that have text or binary data. Of course we are talking about large files or many large files. This is the use case for a distributed file system such as HDFS (hadoop file system) or GFS ( Amazon ). Logically the implementation of distributed file system is similar to say a linux filesystem. You view the file system as a list of blocks of fixed size. On a single node linux file system , all the disk blocks are on one node. In a distributed file system, the blocks are spread across multiple nodes. In HDFS, the name node maintains the metadata for the distributed filesystem -- given a file , which blocks make up the file and which nodes have the blocks. To create a file, the name node may assign a block on a particular node and the client talks directly to service called data node running on the target node to write to block. To read a block, the name node directs the client to the data node that hosts the block and the client reads directly from that block. But the basic algorithm is simple -- break up the file data into blocks and spread them across nodes.
Another example is partitioning of topic logs in Apache Kafka is a messaging system ( they like to call it event streaming) where producers write messages to a topic and consumers read messages from the topic. The storage for a topic is a log file. New messages are appended to the end of log file. They are read from the front. Obviously the logs can grow beyond what can fit on a node. So log is broken into partitions and distributed across multiple nodes. The broker serves producers and consumer. A Kafka cluster has multiple brokers with each broker running on a seprate node managing multiple partitions.
3.0 Rebalancing
Rebalancing is the process of moving partitions between nodes to make the distribution of load even across all the nodes. This is necessary when nodes join or leave the cluster or if the cluster starts receiving more data for certain keys. Either way rebalancing is an expensive operation that needs lot of CPU, memory and network bandwidth. It can have an impact on the performance of regular CRUD processing. In a ideal world, we would like rebalancing to happen automatically behind the scenes, without end users knowing about it. But for performance reasons listed above, that rarely works well in practice. Some databases require an admin to manually start a rebalance, which can be done during a period of low load and monitored.
4.0 Routing
How does a client know which partition to connect to ? The short answer is that the database has to maintain mapping of partition to node. In the case of hash based partitioning, hash maps to a partition which maps to a node. In the case of range based partitioning the key maps to a key range which maps to a partition which maps to a node. The partition node map is available to nodes. If a client can connect to any node, then if the node does not have the partition to handle the clients request, it can redirect the client to the appropriate node or it can get the data from the target node and return to client.
5.0 Use case
There are cases where you might not have a ready made database doing server side partitioning for you and you might need to do it yourself. Or even when the database does it for you, you still need to pick the right partition key for the partitioning to be optimal. Let us look at some large datasets and discuss how they might be logically partitioned.
Let us say you are building a twitter like system.
Say 100 million tweets of 140 character per day
100M * ( 280 bytes + 20 bytes for id, timestamp)
30 GB / day
10 TB / year
Need to store 5 years to data
Need to store and query 50 TB
How does twitter work ?
Users follow other users.
When a user connects, we need to show the most recent tweets from the users he follows.
So we need to store about 50 TB of tweets. Given a user, we need to query say the 50 most recent tweets from the users he follows.
Using commodity hardware, 50TB would need say 25 nodes. What key would you use to partition the data ?
Option 1 : hash based partitioning based on user.
To store tweets, a hash of the user is to used to locate the node where the tweet is stored. To query - for each user that the user follows, use the hash to query the node for that users tweets. A problem with this approach is that some users tweet way more than other users. Their nodes are going to be overloaded while others are idle. Unbalanced load.
Option 2 : hash based on randomly generated tweet id
The problem with this approach is the for every query, you have to query every server and aggregate the results. Inefficient for queries.
Option 3: hash based on timestamp
Timestamp is relevant because for each feed request we want the latest tweets. It would be good if tweets are sorted by timestamp. However with such as hash, at any given point in time , one server is overloaded as all the writes are going to the server.
Option 4:
Given the choices, inefficient query (option 2) is more tolerable that unbalanced load (option 1) which could crash some of the servers making the system unavailable. But we also want queries to return the most recent data (sorted by timestamp). So we can improve querying little bit by combining option 2 and 3. Assume a timestamp in epoch time in increments of 1 sec, the tweetid could be timestamp + auto incremented seq. The sequence gives randomness to the tweet id and will give uniform distribution across node.
So given a epoch 1692547708 you will have tweet ids like
1692547708 1
1692547708 2
1692547708 3
1692547708 n
Of course you are wondering how to generate unique tweet ids in a distributed system. That is a topic for another blog.
6.0 Summary
Partitioning data and spreading it across nodes is fundamental to distributed system. Special thought needs to be given to how the partitioned data can be queried efficiently. Hash based partitioning and range based partitioning are two popular strategies. Nodes can fail or additional nodes may need to added to scale. To ensure that load is even across nodes, partitions may be moved between nodes in a process called rebalancing. For best results design your partition keys so that load is distributed evenly and querying is efficient.
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