In this series of short blog posts we will introduce Kudu’s consistency model, its design and ultimate goals, current features, and next steps. On the way, we’ll shed some light on the more relevant components and how they fit together.
In Part 1 of the series (this one), we’ll cover motivation and design trade-offs, the end goals and the current status.
What is “consistency” and why is it relevant?
In order to cope with ever increasing data volumes, modern storage systems like Kudu have to support many concurrent users while coordinating requests across many machines, each with many threads executing work at the same time. However, application developers shouldn’t have to understand the internal details of how these systems implement this parallel, distributed, execution in order to write correct applications. Consistency in the context of parallel, distributed systems roughly refers to how the system behaves in comparison to a single-machine, single-thread system. In a single-threaded, single-machine storage system operations happen one-at-a-time, in a clearly defined order, making correct applications easy to code and reason about. A developer writing an application against such a system doesn’t have to care about how simultaneous operations interact or about ordering anomalies, so the code is simpler, but more importantly, cognitive load is greatly reduced, freeing focus for the application logic itself.
While such a simple system is definitely possible to build, it wouldn’t be able to cope with very large amounts of data. In order to deal with big data volumes and write throughputs modern storage systems like Kudu are designed to be distributed, storing and processing data across many machines and cores. This means that many things happen simultaneously in the same and different machines, that there are more moving parts and thus more oportunity for mis-orderings and for components to fail. How far systems like Kudu go (or don’t go) in emulating the simple single-threaded, single-machine system a distributed, parallel setting where failures are common is roughly what is referred to as how “consistent” the system is.
Consistency as a term is somewhat overloaded in the distributed systems and database communities, there are many different models, properties, different names for the same concept, and often different concepts under the same name. This post is not meant to introduce these concepts as there are excellent references already available elsewhere (we recommend Kyle Kinsbury’s excellent series of blog posts on the matter, like this one). Throughout this and follow-up posts we’ll refer to consistency loosely as the C in CAP in some cases and as the I in ACID in others; we’ll try to be specific when relevant.
Design decisions, trade-offs and motivation
Consistency is essentially about ordering and ordering usually has a cost. Distributed storage system design must choose to prioritize some properties over others according to the target use cases. That is, trade-offs must be made or, borrowing a term from economics, there is “no free lunch”. Different systems choose different trade-off points; for instance, systems inspired by Dynamo, usually favor availability in the consistency/availability trade-off: by allowing a write to a data item to succeed even when a majority (or even all) of the replicas serving that data item are unreachable, Dynamo’s design is minimizing insertion errors and insert latency (related to availability) at the cost having to perform extra work for value reconciliation on reads and possibly returning stale or disordered values (related to consistency). On the other end of the spectrum, traditional DBMS design is often driven by the need to support transactions of arbitrary complexity while providing the users stronger, predictable, semantics, favoring consistency at the cost of scalability and availability.
Kudu’s overarching goal is to enable fast analytic workloads over large amounts of mutable data, meaning it was designed to perform fast scans over large volumes of data stored in many servers. In practical terms this means that, when given a choice, more often than not, we opted for the design that would enable Kudu to have faster scan performance (i.e. favoring reads even if it meant pushing a bit more work to the path that mutates data, i.e. writes). This does not mean that the write path was not a concern altogether. In fact, modern storage systems like Google’s Spanner global-scale database demonstrate that, with the right set of trade-offs, it is possible to have strong consistency semantics with write latencies and overall availability that are adequate for most use cases (e.g. Spanner achieves 5 9’s of availability). For the write path, we often made similar choices in Kudu.
Another important aspect that directed our design decisions is the type of write workload we targeted. Traditionally, analytical storage systems target periodic bulk write workloads and a continuous stream of analytical scans. This design is often problematic in that it forces users to have to build complex pipelines where data is accumulated in one place for later loading into the storage system. Moreover, beyond the architectural complexity, this kind of design usually also means that the data that is available for analytics is not the most recent. In Kudu we aimed for enabling continuous ingest, i.e. having a continuous stream of small writes, obviating the need to assemble a pipeline for data accumulation/loading and allowing analytical scans to have access to the most recent data. Another important aspect of the write workloads that we targeted in Kudu is that they are append-mostly, i.e. most insert new values into the table, with a smaller percentage updating currently existing values. Both the average write size and the data distribution influence the design of the write path, as we’ll see in the following sections.
One last concern we had in mind is that different users have different needs when it comes to consistency semantics, particularly as it applies to an analytical storage system like Kudu. For some users consistency isn’t a primary concern, they just want fast scans, and the ability to update/insert/delete values without needing to build a complex pipeline. For example, many machine learning models are mostly insensitive to data recency or ordering so, when using Kudu to store data that will be used to train such a model, consistency is often not as primary a concern as read/write performance is. In other cases consistency is a much higher priority. For example, when using Kudu to store transaction data for fraud analysis it might be important to capture if events are causally related. Fraudulent transactions might be characterized by a specific sequence of events and when retrieving that data it might be important for the scan result to reflect that sequence. Kudu’s design allows users to make a trade-off between consistency and performance at scan time. That is, users can choose to have stronger consistency semantics for scans at the penalty of latency and throughput or they can choose to weaken the consistency semantics for an extra performance boost.
Kudu currently lacks support for atomic multi-row mutation operations (i.e. mutation operations to more than one row in the same or different tablets, planned as a future feature). So, when discussing writes, we’ll be talking about the consistency semantics of single row mutations. In this context we’ll discuss Kudu’s properties more from a key/value store standpoint. On the other hand Kudu is an analytical storage engine so, for the read path, we’ll also discuss the semantics of large (multi-row) scans. This moves the discussion more into the field of traditional DBMSs. These ingredients make for a non-traditional discussion that is not exactly apples-to-apples with what the reader might be familiar with, but our hope is that it still provides valuable, or at least interesting, insight.
Consistency options in Kudu
Consistency, as well as other properties, are underpinned in Kudu by the concept of a timestamp. In follow-up posts we’ll look into detail how these are assigned and how they are assembled. For now it’s sufficient to know that a timestamp is a single, usually large, number that has some mapping to wall time. Each mutation of a Kudu row is tagged with one such timestamp. Globally, these timestamps form a partial order over all the rows with the particularity that causally related mutations (e.g. a write mutation that is the result of the value obtained from a previous read) may be required to have increasing timestamps, depending on the user’s choices.
Row mutations performed by a single client instance are guaranteed to have increasing timestamps
thus reflecting their potential causal relationship. This property is always enforced. However
there are two major “knobs” that are available to the user to make performance trade-offs, the
Read mode, and the
External Consistency mode (see here
for more information on how to use the relevant APIs).
The first and most important knob, the
Read mode, pertains to what is the guaranteed recency of
data resulting from scans. Since Kudu uses replication for availability and fault-tolerance, there
are always multiple replicas of any data item.
Not all replicas must be up-to-date so if the user cares about recency, e.g. if the user requires
that any data read includes all previously written data from a single client instance then it must
READ_AT_SNAPSHOT read mode. With this mode enabled the client is guaranteed to observe
“READ YOUR OWN WRITES” semantics, i.e. scans from a client will always include all previous mutations
performed by that client. Note that this property is local to a single client instance, not a global
The second “knob”, the
External Consistency mode, defines the semantics of how reads and writes
are performed across multiple client instances. By default,
External Consistency is set to
CLIENT_PROPAGATED, meaning it’s up to the user to coordinate a set of timestamp tokens with clients (even
across different machines) if they are performing writes/reads that are somehow causally linked.
If done correctly this enables STRICT SERIALIZABILITY, i.e. LINEARIZABILITY and
SERIALIZABILITY at the same time, at the cost of having the user coordinate the timestamp
tokens across clients (a survey of the meaning of these, and other definitions can be found
The alternative setting for
External Consistency is to have it set to
COMMIT_WAIT (experimental), which guarantees the same properties through a different means, by
implementing Google Spanner’s TrueTime. This comes at the cost of higher latency (depending on how
tightly synchronized the system clocks of the various tablet servers are), but doesn’t require users
to propagate timestamps programmatically.
In following posts we’ll look into the several components of Kudu’s architecture that come together to enable the consistency semantics introduced in the previous section, including:
- Transactions and the Transaction Driver
- Concurrent execution with Multi-Version Concurrency Control
- Exactly-Once semantics with Replay Cache
- Replication, Crash Recovery with Consensus and the Write-Ahead-Log
- Time keeping and timestamp assignment
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