Future Technology

Migrating Essential Site visitors At Scale with No Downtime — Half 2 | by Netflix Know-how Weblog | Could, 2023

10 min read

Shyam Gala, Javier Fernandez-Ivern, Anup Rokkam Pratap, Devang Shah

Image your self enthralled by the newest episode of the one you love Netflix collection, delighting in an uninterrupted, high-definition streaming expertise. Behind these excellent moments of leisure is a posh mechanism, with quite a few gears and cogs working in concord. However what occurs when this equipment wants a metamorphosis? That is the place large-scale system migrations come into play. Our earlier weblog submit introduced replay site visitors testing — a vital instrument in our toolkit that enables us to implement these transformations with precision and reliability.

Replay site visitors testing offers us the preliminary basis of validation, however as our migration course of unfolds, we’re met with the necessity for a fastidiously managed migration course of. A course of that doesn’t simply reduce threat, but in addition facilitates a steady analysis of the rollout’s affect. This weblog submit will delve into the methods leveraged at Netflix to introduce these adjustments to manufacturing.

Canary deployments are an efficient mechanism for validating adjustments to a manufacturing backend service in a managed and restricted method, thus mitigating the danger of unexpected penalties that will come up because of the change. This course of includes creating two new clusters for the up to date service; a baseline cluster containing the present model operating in manufacturing and a canary cluster containing the brand new model of the service. A small share of manufacturing site visitors is redirected to the 2 new clusters, permitting us to observe the brand new model’s efficiency and examine it in opposition to the present model. By gathering and analyzing key efficiency metrics of the service over time, we are able to assess the affect of the brand new adjustments and decide in the event that they meet the supply, latency, and efficiency necessities.

Some product options require a lifecycle of requests between the shopper machine and a set of backend providers to drive the function. For example, video playback performance on Netflix includes requesting URLs for the streams from a service, calling the CDN to obtain the bits from the streams, requesting a license to decrypt the streams from a separate service, and sending telemetry indicating the profitable begin of playback to one more service. By monitoring metrics solely on the degree of service being up to date, we would miss capturing deviations in broader end-to-end system performance.

Sticky Canary is an enchancment to the normal canary course of that addresses this limitation. On this variation, the canary framework creates a pool of distinctive buyer gadgets after which routes site visitors for this pool constantly to the canary and baseline clusters all through the experiment. Aside from measuring service-level metrics, the canary framework is ready to hold observe of broader system operational and buyer metrics throughout the canary pool and thereby detect regressions on your entire request lifecycle circulate.

Sticky Canary

It is very important be aware that with sticky canaries, gadgets within the canary pool proceed to be routed to the canary all through the experiment, doubtlessly leading to undesirable conduct persisting by means of retries on buyer gadgets. Due to this fact, the canary framework is designed to observe operational and buyer KPI metrics to detect persistent deviations and terminate the canary experiment if needed.

Canaries and sticky canaries are worthwhile instruments within the system migration course of. In comparison with replay testing, canaries enable us to increase the validation scope past the service degree. They allow verification of the broader end-to-end system performance throughout the request lifecycle for that performance, giving us confidence that the migration is not going to trigger any disruptions to the shopper expertise. Canaries additionally present a possibility to measure system efficiency beneath completely different load situations, permitting us to determine and resolve any efficiency bottlenecks. They allow us to additional fine-tune and configure the system, guaranteeing the brand new adjustments are built-in easily and seamlessly.

A/B testing is a well known methodology for verifying hypotheses by means of a managed experiment. It includes dividing a portion of the inhabitants into two or extra teams, every receiving a unique therapy. The outcomes are then evaluated utilizing particular metrics to find out whether or not the speculation is legitimate. The business regularly employs the method to evaluate hypotheses associated to product evolution and person interplay. Additionally it is extensively utilized at Netflix to check adjustments to product conduct and buyer expertise.

A/B testing can also be a worthwhile software for assessing important adjustments to backend methods. We will decide A/B take a look at membership in both machine utility or backend code and selectively invoke new code paths and providers. Throughout the context of migrations, A/B testing allows us to restrict publicity to the migrated system by enabling the brand new path for a smaller share of the member base. Thereby controlling the danger of surprising conduct ensuing from the brand new adjustments. A/B testing can also be a key method in migrations the place the updates to the structure contain altering machine contracts as nicely.

Canary experiments are sometimes performed over durations starting from hours to days. Nevertheless, in sure cases, migration-related experiments could also be required to span weeks or months to acquire a extra correct understanding of the affect on particular High quality of Expertise (QoE) metrics. Moreover, in-depth analyses of specific enterprise Key Efficiency Indicators (KPIs) might require longer experiments. For example, envision a migration situation the place we improve the playback high quality, anticipating that this enchancment will result in extra prospects partaking with the play button. Assessing related metrics throughout a substantial pattern dimension is essential for acquiring a dependable and assured analysis of the speculation. A/B frameworks work as efficient instruments to accommodate this subsequent step within the confidence-building course of.

Along with supporting prolonged durations, A/B testing frameworks supply different supplementary capabilities. This method allows take a look at allocation restrictions based mostly on components similar to geography, machine platforms, and machine variations, whereas additionally permitting for evaluation of migration metrics throughout comparable dimensions. This ensures that the adjustments don’t disproportionately affect particular buyer segments. A/B testing additionally gives adaptability, allowing changes to allocation dimension all through the experiment.

We’d not use A/B testing for each backend migration. As an alternative, we use it for migrations by which adjustments are anticipated to affect machine QoE or enterprise KPIs considerably. For instance, as mentioned earlier, if the deliberate adjustments are anticipated to enhance consumer QoE metrics, we might take a look at the speculation by way of A/B testing.

After finishing the varied phases of validation, similar to replay testing, sticky canaries, and A/B checks, we are able to confidently assert that the deliberate adjustments is not going to considerably affect SLAs (service-level-agreement), machine degree QoE, or enterprise KPIs. Nevertheless, it’s crucial that the ultimate rollout is regulated to make sure that any unnoticed and surprising issues don’t disrupt the shopper expertise. To this finish, we have now applied site visitors dialing because the final step in mitigating the danger related to enabling the adjustments in manufacturing.

A dial is a software program assemble that permits the managed circulate of site visitors inside a system. This assemble samples inbound requests utilizing a distribution operate and determines whether or not they need to be routed to the brand new path or stored on the present path. The choice-making course of includes assessing whether or not the distribution operate’s output aligns inside the vary of the predefined goal share. The sampling is completed constantly utilizing a hard and fast parameter related to the request. The goal share is managed by way of a globally scoped dynamic property that may be up to date in real-time. By rising or reducing the goal share, site visitors circulate to the brand new path might be regulated instantaneously.


The choice of the particular sampling parameter depends upon the precise migration necessities. A dial can be utilized to randomly pattern all requests, which is achieved by deciding on a variable parameter like a timestamp or a random quantity. Alternatively, in situations the place the system path should stay fixed with respect to buyer gadgets, a relentless machine attribute similar to deviceId is chosen because the sampling parameter. Dials might be utilized in a number of locations, similar to machine utility code, the related server part, and even on the API gateway for edge API methods, making them a flexible software for managing migrations in complicated methods.

Site visitors is dialed over to the brand new system in measured discrete steps. At each step, related stakeholders are knowledgeable, and key metrics are monitored, together with service, machine, operational, and enterprise metrics. If we uncover an surprising challenge or discover metrics trending in an undesired route throughout the migration, the dial offers us the aptitude to rapidly roll again the site visitors to the previous path and deal with the difficulty.

The dialing steps may also be scoped on the knowledge heart degree if site visitors is served from a number of knowledge facilities. We will begin by dialing site visitors in a single knowledge heart to permit for a better side-by-side comparability of key metrics throughout knowledge facilities, thereby making it simpler to look at any deviations within the metrics. The length of how lengthy we run the precise discrete dialing steps may also be adjusted. Operating the dialing steps for longer durations will increase the likelihood of surfacing points that will solely have an effect on a small group of members or gadgets and may need been too low to seize and carry out shadow site visitors evaluation. We will full the ultimate step of migrating all of the manufacturing site visitors to the brand new system utilizing the mixture of gradual step-wise dialing and monitoring.

Stateful APIs pose distinctive challenges that require completely different methods. Whereas the replay testing method mentioned within the earlier a part of this weblog collection might be employed, further measures outlined earlier are needed.

This alternate migration technique has confirmed efficient for our methods that meet sure standards. Particularly, our knowledge mannequin is straightforward, self-contained, and immutable, with no relational facets. Our system doesn’t require strict consistency ensures and doesn’t use database transactions. We undertake an ETL-based dual-write technique that roughly follows this sequence of steps:

  • Preliminary Load by means of an ETL course of: Information is extracted from the supply knowledge retailer, remodeled into the brand new mannequin, and written to the newer knowledge retailer by means of an offline job. We use customized queries to confirm the completeness of the migrated data.
  • Steady migration by way of Twin-writes: We make the most of an active-active/dual-writes technique to migrate the majority of the information. As a security mechanism, we use dials (mentioned beforehand) to regulate the proportion of writes that go to the brand new knowledge retailer. To keep up state parity throughout each shops, we write all state-altering requests of an entity to each shops. That is achieved by deciding on a sampling parameter that makes the dial sticky to the entity’s lifecycle. We incrementally flip the dial up as we acquire confidence within the system whereas fastidiously monitoring its total well being. The dial additionally acts as a change to show off all writes to the brand new knowledge retailer if needed.
  • Steady verification of data: When a report is learn, the service reads from each knowledge shops and verifies the useful correctness of the brand new report if present in each shops. One can carry out this comparability reside on the request path or offline based mostly on the latency necessities of the actual use case. Within the case of a reside comparability, we are able to return data from the brand new datastore when the data match. This course of offers us an thought of the useful correctness of the migration.
  • Analysis of migration completeness: To confirm the completeness of the data, chilly storage providers are used to take periodic knowledge dumps from the 2 knowledge shops and in contrast for completeness. Gaps within the knowledge are crammed again with an ETL course of.
  • Lower-over and clean-up: As soon as the information is verified for correctness and completeness, twin writes and reads are disabled, any consumer code is cleaned up, and skim/writes solely happen to the brand new knowledge retailer.
Migrating Stateful Methods

Clear-up of any migration-related code and configuration after the migration is essential to make sure the system runs easily and effectively and we don’t construct up tech debt and complexity. As soon as the migration is full and validated, all migration-related code, similar to site visitors dials, A/B checks, and replay site visitors integrations, might be safely faraway from the system. This consists of cleansing up configuration adjustments, reverting to the unique settings, and disabling any momentary parts added throughout the migration. As well as, you will need to doc your entire migration course of and hold data of any points encountered and their decision. By performing a radical clean-up and documentation course of, future migrations might be executed extra effectively and successfully, constructing on the teachings discovered from the earlier migrations.

We’ve got utilized a variety of methods outlined in our weblog posts to conduct quite a few giant, medium, and small-scale migrations on the Netflix platform. Our efforts have been largely profitable, with minimal to no downtime or important points encountered. All through the method, we have now gained worthwhile insights and refined our methods. It ought to be famous that not the entire methods introduced are universally relevant, as every migration presents its personal distinctive set of circumstances. Figuring out the suitable degree of validation, testing, and threat mitigation requires cautious consideration of a number of components, together with the character of the adjustments, potential impacts on buyer expertise, engineering effort, and product priorities. Finally, we goal to attain seamless migrations with out disruptions or downtime.

In a collection of forthcoming weblog posts, we are going to discover a number of particular use circumstances the place the methods highlighted on this weblog collection have been utilized successfully. They are going to deal with a complete evaluation of the Advertisements Tier Launch and an intensive GraphQL migration for numerous product APIs. These posts will supply readers invaluable insights into the sensible utility of those methodologies in real-world conditions.

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