Hydra—Kubernetes based Dataset PubSub and Volume Management System

A generic solution to data management for services in Kubernetes

In the previous articles of this advent series, we described the architecture of our search engine and highlighted technologies which help us serve search results.

After many iterations on our architecture, we are finally on a microservice architecture pattern, driven through a container orchestration platform: Kubernetes. This brought us many engineering improvements, but it also has introduced new challenges, such as, the ability to perform a dataset propogation via Pub-Sub, along with volume management for microservices. Let us illustrate this point further by example.

Scenario—Working with Services and Datasets

  • Let us say, a team of engineers are developing a service, which has a dependency on dataset X, Y and Z.
  • The service requires that periodically, these datasets are updated (Data Updates).
  • Typically, a dataset will be versioned. Hence, there is the need for a versioning scheme, given the frequency of updates, time-stamps are a good option. e.g.: 2019-12-15T00-00-00 i.e. YYYY-MM-DDThh-mm-ss (Data Versioning).
  • For the service to present a holistic view of the system the datasets in a given version can have interdependencies (Service-Data State Management).
  • Newer datasets can be published periodically through an automated pipeline and artifacts stored in an object store like S3.
  • One then needs to keep track of when these datasets finish building and thereby notify the downstream service of their availability (keeping in mind that all interdependencies must be satisfied).
  • Once all three datasets are available, we push the data to instances where services are running (Data provisioning on Volumes).
  • The data is downloaded behind the scenes, without affecting the downstream service.
  • Eventually, the service gets notified of successful download for all the datasets on the instance.
  • Service intrinsically identifies and verifies this change and shifts to new datasets incrementally avoiding downtime…

This is a very common pattern for services, which rely on frequently updating immutable datasets. If this is a one-off scenario, for instance a prototype or a service whose data updates once every 3 months, then you are probably better off doing all steps necessary by hand or by writing a custom automation scripts. But, if this is a regular scenario like in the real world—for example when dealing with multiple services, with a multitude of datasets, each having their own periodicity—it may lead to potentially severe issues pretty quickly.

Moreover, there can be two major failure scenarios:

  1. Pods can be evicted (Kubernetes Specific).
  2. Instances may be lost (General Scenario).

Hence, centrally automating dataset propagation, in a high failure system has its challenges but brings in considerable value.

In this post, we will briefly introduce a solution built in-house, named Hydra, which provides a generic solution to the use-case mentioned above. We describe the need for such a system in our organization.

On a high level, Hydra is composed of a set of components which,

  1. Provide a dataset pub-sub mechanism.
  2. Maintain a global state.
  3. Manage datasets on volumes for services running within a Kubernetes cluster.

At its core, Hydra is able to tap into the Kubernetes service registry for discovery and observability of running services.

Managing lifecycle of datasets (models and indexes) on Instances

One of the core requirements of our search engine is fast access to different types of datasets which can have both static or dynamic composition. The dynamism of datasets can be described w.r.t their update frequency (some update real-time, some based on a triggered event and some based on a pre-configured cron job setting).

The dataset representation can be a machine learned model (Tensorflow, Keras, Scikit-Learn, etc.), a Granne Index (Approximate Nearest Neighbor Index), qpick Index, a key-value compiled dataset (Keyvi, RocksDB, etc.), a service configuration, or an artifact from a batch job. The size may range from a couple of megabytes to several terabytes.

For the purpose of serving search results, the medium and placement of these indexes and models is also important. Datasets may be located on spinning magnetic disks, SSDs or on RAM (ram-disks). This placement differs from service to service, with varying degree of Service Level Objectives (SLO) requirements (access patterns, latency guarantees, replication, etc.) In the context of cloud provisioning, the storage medium can also differ. For some models which do not have strict latency requirements, we choose Elastic Block Storage (EBS) but for others where read latency needs to be minimized and IOPS can be a determining factor to achieve SLOs, we employ local volumes based on raided SSDs using the NVME protocol. For services requiring even more guarantees we rely on large ram-disks.

Every microservice in the critical request path, as explained in our previous article[1], can have a persistent layer composed of the aforementioned datasets. In order to provide fresh results and to support graceful degradation on failures we need to have a solid mechanism for dataset propagation.

Other use cases may include: data exploration while developing new features, running A/B tests or modifying configuration of running services on the fly.

Finally, there is another important reason for solid dataset propagation. If it does not exist, every team working on a feature-related data will have to reinvent the wheel. We have experienced first hand that multiple teams come up with different solutions for downloading and keeping state of their services, this tooling, however, was tightly coupled with the system, making re-usability impossible. A common system is needed to save people the tedious task of “getting ready”.

Introducing Hydra

These requirements presented an opportunity to centralize said effort and bake it with infrastructure support. A centralized effort also brings in an opportunity to build better observability and fault detection tools. Knowing the actual placement of the data as well as the ability to quickly change it, is vital for a working system, where the data is served from and facilitates quick changes. The service which solves these challenges for us is called Hydra.

The core constituents of Hydra are as follows:

  • It provides a generic pub-sub approach to handling datasets at scale and aids a service to publish or subscribe to versioned datasets.
  • Introduces a concept of channels, publishers and consumers.
  • A channel contains an ordered list of releases where the head pointer points to the latest release subscribers (services) would be interested in. Channel states are managed by a Global Manager and metadata for the same is maintained in a highly available Consul.
  • Publishers can update a channel with a new release (HTTP API) and consumers will be notified of new changes.
  • Data update diffs are propagated from Global Manager to downstream agents named Docto running as deamon-sets which can ensure a global state (downloading of data from S3 and cleanup) in a reconciliation loop.
  • On successful availability of a release on an instance, consumers are incrementally notified of newer datasets, which are then rolled out. This is done via an additional component named AppMan which is responsible for this task. This allows services to be agnostic of Hydra’s workings.
  • Newer datasets can be rolled out using the readiness probe feature from Kubernetes. Whereby given a service is in high availability, rolling data deployments can be done by draining traffic from a pod using readiness probes.
  • In scenarios of replication, this is done one by one for a given service, thereby reducing downtime.
  • The datasets are immutable in nature i.e. a single release represents a complete view of data and incurs no dependency on previous versioned data releases in the same channel.

Working with Hydra—End-to-End Use Case

1. Publishing State of a Release

A user or a service can post the dataset source paths (composed within a release) using client-side libraries / CLI and its storage placement as required with the help of a data template, provided as part of Hydra’s client CLI tool. This complete information is posted via the HTTP API to a Global Manager Server.

DISK = 0
RAM = 1

    "dataset_1": {
        "SourcePath": f"s3://some-bucket/some-data-set-1/{VERSION_PLACEHOLDER}/file1.xyz",
        "Placement": DISK
    "dataset_2": {
        "SourcePath": f"s3://some-bucket/some-data-set-2/{VERSION_PLACEHOLDER}/file2.xyz",
        "Placement": RAM

Where, SourcePath is the original S3 bucket path where the data is / will be available, and Placement describes where the data must be eventually provisioned on the instance.

Some extra information like Environment, Release and Namespace are also required by the Hydra CLI client. It allows Hydra to formulate channel identity and scoping and is stored as metadata for respective channel. This can be propagated using special flags:

--env: Environment describes which cluster to target
--release: Release name of service relevant to dataset template
-n: Namespace of deployed service

The hydra client submits this data template with the VERSION_PLACEHOLDER computed based on two factors:

  1. Identifying most recent datasets available on S3.
  2. If datasets should have the same timestamps, then it verifies the same before proceeding ahead.

We assume that once datasets are available on S3 they have been already verified for integrity and are fit to serve. These checks are done as part of the data-pipeline’s post processing step.

Channels can have retention policies on the number of releases to keep. Also, you can configure the number of actual releases to keep on instances. The default value is 2. This means that apart from the newly published release, the previous dataset is also preserved on instances. All older releases can be cleaned up based on retention policies to improve disk utilization.

2. Hydra’s Global Manager & Data Download (S3 -> Instance)

Once a new data release is published to a channel, the global manager is able to correlate consumers based on discovery from the Kubernetes service registry or explicit registration.

All the instances in the cluster run an agent as a daemon-set which only communicates with the Global manager and is wholly responsible to download the models from S3 to instance. Thus the global manager communicates with this daemon application and sends information about which datasets to download. The daemon verifies the requests, identifies the placement, checks if there is enough storage available on the instance and starts the download. It constantly communicates the progress of the download with Global Manager and notifies a SUCCESS upon its completion.

3. Application Dataset Reload

Next, the Global Manager notifies the consumer through client-side library interfaces on availability of newer release datasets. This is done incrementally to avoid downtimes. This initiates the process, where at first readiness probes are used to drain traffic, then either by using a built-in mechanism or a service restart, new data is reloaded for the service, and readiness probes are turned back ON, to serve live traffic.

If there are several replica pods behind a service, the Global manager performs the reload one by one, making sure each pod comes back up before starting the data reloads in other pods of the same service.


  1. In development settings, for non critical data services we employ a downscaler strategy which downscales deployments to zero during non working hours. This constitutes an uptime during the following times: Mon-Fri 08:00-19:30 Europe/Berlin.

    This means that not only the container, but instance with data is downscaled. But, since hydra is autonomous, we can quickly prepare the instance with the data when the deployment is scaled up again on a newer instance. This works great for us, as we save some costs by not spinning high capacity instances (a necessary requirement for hosting search indexes) in development setting during non-working hours.

  2. In a special case where RAM is the preferred placement medium, given the fact that they are usually a scarce and expensive resource, we make sure that they are also copied over to disk if available for faster load times to RAMDISK. We only keep a single version on RAM for that dataset. Due to locking on older datasets or limited availability of space on-ramdisk services can be drained and shifted to newer releases.

Future Work

Hydra shares a lot of its DNA with project Gutenberg, which was announced two months ago.

We will keep an eye on Gutenberg’s development since both systems attempt to solve the very same problems. It is just natural that different teams reach similar conclusions when presented with the same problem. Nonetheless, it is always nice to get external validation of the decision taken.

We have been developing Hydra for the last 2 years and gradually integrating it into our production systems. However, it is far from a finished project. A peak on the project’s future timeline:

  • Better control over Volume discovery, provisioning, budgeting and isolation.
  • Multi language support for client side libraries. Including Rust and Go. Currently we only provide Python.
  • More scoping options such as region and pinning releases to a specific version.
  • Encryption and access control to improve security footprint and avoid disaster scenarios.
  • Better incremental rollouts by introducing new strategies like disruption budgets and surge protections.
  • Improvements in observability for service owners.
  • Better channel retention strategies, such as time based retention.


At Cliqz, Hydra runs in production and handles data for a variety of data-hungry services deployed in our search stack. It drives part of our search index which is updated weekly (window-based) as part of our multi-tier lambda architecture[1:1].

Through this post, we wanted to highlight the common challenges in data management and how we solve them using Hydra. We plan to share more details in future posts.

Hydra is still under active development and will eventually be open sourced. We believe, that the Kubernetes and ML community can greatly benefit from an end-to-end model management versioning and propagation system.

Footnotes and Remarks

  1. The Architecture of a Large-Scale Web Search Engine, circa 2019. ↩︎ ↩︎