Introduction
This document aims to summarise the outcome of the recent performance testing conducted by the PTF team and provide some suggestions as to how we might improve the performance of checking out an item under load.
Context
History
Development of modules within the circulation domain started early in FOLIO's overall development, meaning they are some of the oldest modules and integrate heavily with other older modules.
Historical Constraints
When FOLIO started, it began with some constraints that were applied when these modules were developed.
I've picked out a few that could be relevant to how we got to the current design
- Business logic must use the most current state for decisions (this is what the SME have told me in the past conversations I've had and is supported in technical documentation)
- Business logic and storage are split in two separate modules (in order to support independent substitution)
- All integration between modules is done via HTTP APIs (proxied via Okapi)
- All data is stored within PostgreSQL
- A record oriented design with a single system of record for each record type (business logic / storage separation not-withstanding)
As these weren't explicitly documented at the time, it is difficult to know if they have changed over time.
There are counter examples which would suggest that they may have changed:
- some more recent modules (e.g. mod-courses, mod-data-export-spring) have both business logic and persistent storage within the same module
- some modules (e.g. mod-search, mod-inventory, mod-data-export-spring) use Kafka (rather than HTTP) for communication
- some modules (e.g. mod-search) use Elastic Search for persistent storage (though not the book of record)
I don't know if these constitute a change in policy or specific exceptions.
Some of the options presented below contradict these constraints and would need them to change to be tolerable and coherent within FOLIO's architecture.
Expectations
A checkout (including the staff member scanning the item barcode) must complete within 1 second (from the documented /wiki/spaces/DQA/pages/2658550). It is stated that this includes the time for a person to scan the item barcode.
For the purposes of this analysis I shall assume the following (neither of which are likely true in practice):
- None of this time is taken up by the human scanning the barcode (and interacting with the UI)
- None of this time is taken up the FOLIO UI (in practice, the UI has to fetch item information to potentially ask the staff member some questions)
The performance requirements do not provide any guidance on what the conditions (load parameters or resource configuration) this expectation should hold for.
Thus for the purposes of this analysis, the expectation is that:
the check out API must respond within 1 second under load from 8 concurrent requests (with no tolerance for outliers that exceed this limit)
Solution Constraints
Beyond the general constraints on architectural decisions (listed above), I've imposed the following constraints to limit the option space:
- No changes to the client interface of the circulation APIs
- excludes using hypermedia controls to defer fetching some data
- Only existing infrastructure can be used (I'm including Kafka in this, even though it isn't official yet)
- excludes options which including changing the storage technology e.g. MongoDB or integration mechanism e.g. gRPC
Analysis
Limitations of Analysis
Thanks to the PTF team we have overall performance data for (an approximation of) the whole check out process.
However we only have a single detailed sample (with less data and no load) of the downstream requests from the check out API. That sample is not representative of the range of response times likely present in a whole performance test run using more realistic load parameters.
Thus, this analysis has to assume that the sample is representative whilst also interpreting it skeptically (because it is likely far more optimistic than the heavier load scenarios).
We also do not know:
- why the response times of the constituent parts do not equate to the overall response time
- what amount of time Okapi takes to process requests / responses
- what amount of time mod-circulation takes to use this information to make decisions e.g. to apply the circulation rules
This factors mean it is challenging to draw reliable and specific conclusions about the requests involved, meaning that most of the analysis will be broad and general.
What takes up the time?
| Step | Time Take |
|---|---|
| Generating a downstream token (assumed to be once per incoming request) | 133 ms (99 + 6 + 16 + 12) |
| Checking request token (for each downstream request) | 12ms (average) |
| Downstream request | 50ms (average) |
There are 27 downstream requests triggered by mod-circulation during the sample check out.
Once we deduct the initial overhead (133ms) that leaves us with an approximate budget of 32ms per request (867 ms / 27).
At the moment, the average request in our low load sample takes 62ms (including proxying overhead). This is more than double the budget we have available and we can expect the situation to be worse under load.
Whilst there are some outliers (that are still likely lower than under load numbers) that push up this number, I think this indicates the degree of challenge we have with the current approach.
What could we do?
Broadly speaking there are three things that can be done to improve the response time of a check out API request
- Reduce the amount of time each request takes
- Make downstream requests concurrently
- Reduce the quantity of downstream requests made
These ideas will be the framing for the proposal part of this document.
Options
Improve the performance of individual downstream requests
Characteristics
- Scope for improvement is limited as many of these requests are individually relatively fast
- Improvements are brittle and can be easily undone by changes to downstream modules (and it may take a while to become aware of degradation)
- Limited by the constraints of the downstream modules (e.g. the data is currently stored as JSONB)
- May involve changes in multiple modules
- Retains the same amount of downstream requests
- Retains the same overhead from Okapi proxying
Make downstream requests concurrently
For example, once the item is received, the locations, loan types and material types can be fetched concurrently.
Characteristics
- Only involves changes to mod-circulation
- Increases the complexity of the code in mod-circulation
- Not all requests can be made concurrently (some are based upon prior requests or decisions that cannot be made up front)
- Is likely limited by how well other modules / database can handle concurrent requests
- Retains the same overall load on the system as before (although it may be compressed in time)
- Retains the same amount of downstream requests
- Retains the same overhead from Okapi proxying
Combine multiple downstream requests for related records into a single request
Introduces context-specific APIs that are intended for specific use. At most, this can only be applied to the requests made to the same module.
It may not make sense to combine all of the record types from a single module. For example, does it make sense to have an API that fetches existing open loans and loan policies together?
We are already introducing a new API in mod-inventory-storage in this manner to improve the pre-checks made by the check out UI.
Characteristics
- Reduces the amount of individual downstream requests (and hence the Okapi proxying overhead)
- Requires at least one downstream request per destination module
- Requires at least one database query per downstream module
- Might reduce the response time off the downstream request (compared to the combination of )
- Might reduce the load on downstream modules (depending upon how the combined request is handled, it is possible the load increases)
- Reduction in downstream requests is limited to number of record types within a single module
- Increases the amount of APIs to maintain (what I call the surface area of the module)
- Increases the coupling between modules (by introducing the clients context into the other module)
- Increases the coupling between the record types involved (e.g. it's harder to move record types to other modules when they are included in APIs together, changes to them ripple across APIs)
Use copies of data to make decisions
In order for it to be acceptable, a variety of stakeholders within the community would need to accept some tolerance for decisions being made with stale information. When I've talked to folks about this previously, they have been uncomfortable with doing this (see above).
Characteristics
- Requires no downstream requests for fetching data during check out process
- Increases the potential for stale data to be used for decisions
- Is contrary to constraints that may still be present in FOLIO
- Introduces complexity of processing messages and persistent storage into mod-circulation
- Introduces a dependency on a database from mod-circulation
- Introduces a dependency on messages produced by other modules
- State changes still require a downstream request (and the requisite proxying overhead)
Variations
The characteristics of this approach varies more based upon some design decisions we make. A couple of the significant ones are outlined below.
These are only a very high level comparison of the characteristics, there are lots of alternative designs in both of these categories that lead to different characteristics.
Where is the data kept?
| Memory | PostgreSQL | |
|---|---|---|
| Volatility | lost when the module instance is terminated | retained even if module instances are terminated |
| Locality | local copies for each module instance | shared between module instances |
| Access Control | shared needs to be controlled with code within the module | can be controlled using mechanisms provided by the database server |
| Responsiveness | Likely faster if cached value is present, likely slower if not | Dependent upon network and database load |
| Record Type Suitability | Better suited to smaller sets that change rarely, e.g. reference types | Can be used for any kind of record type |
| Infrastructure needs | None | Requires a database for mod-circulation |
How is the copied data updated?
| Periodic HTTP requests | Messages consumed from Kafka | |
|---|---|---|
| Freshness | Dependent upon frequency of periodic refresh. Likely to be lead to data being stale for longer than with messaging | Dependent upon message processing latency |
| Access requirements | Needs a system user or module permissions granting to a timer endpoint | Needs access to Kafka topics for every record type (assuming record snapshot based messages as used with mod-search) |
| Initial population / manual state refresh | Requires requests to fetch all records for for all cached records types | Either requires reprocessing of persistent topic (not currently allowed by FOLIO standards) or custom process (similar to mod-search re-index process) |
| Load on other modules during synchronisation | Could be significant. Dependent upon number of record types and quantity of records | Potentially none with persistent topics (not currently allowed by FOLIO standards) |
| Freshness measurement | ||
Combine the business logic and storage modules together
Characteristics
- Removes all downstream for record types within the circulation domain e.g. loans, requests, loan policies etc (include state changes e.g. creating a loan, fulfilling a request)
- Removes the distinction between business logic and storage representations of those records types
- Allows for state changes within the circulation domain to be done within a database transaction
- Is contrary to constraints that may still be present in FOLIO
- Storage modules have been used to workaround cyclic dependencies constraints in Okapi, removing them might involve changing other modules to avoid this in other ways
Recommendations
Before reading this, please take some time to consider the options presented above and consider them for yourself (alongside any others you may think of) in order to reduce the potential for my recommendations to sway your opinion.
There are lots of unknowns with all of these options. It is difficult to predict how long they will take or how much improvement will be achieved. Please keep that in mind when considering these recommendations.
Earliest reasonable improvement
Combining multiple downstream requests into a single request is likely to provide some improvement. This work is familiar to developers and can be achieved without contradicting broader architectural concerns (beyond the coupling considerations).
As there is already ongoing development work that explores this, we can use that to gauge the effectiveness of this approach before committing to a direction for continued work.
Most significant improvement
Copying data into circulation has the most potential for improvement as it removes the need for many of the downstream requests entirely.
However, this work requires:
- adoption of techniques (e.g. synchronising copied data, messaging, caching) and technologies (e.g. Kafka) unfamiliar to most developers in FOLIO
- agreement from many stakeholders (e.g. SME's, TC) that it is acceptable to use potentially stale data for making decisions
Appendices
Definitions
| Phrase | Definition |
|---|---|
| Downstream request | A request made by a module (via Okapi) in order to fulfil the original incoming request e.g. mod-circulation makes a request to mod-users to fetch patron information |
| Response time | The time taken from the client making the request to receiving a response |
Requests made during a typical check out
The first 4 lines of the table describe the initial requests made by Okapi in reaction to the incoming request (to check out). I believe there are circumstances where these requests are made again, however that is omitted from this analysis.
| Intent | Endpoint | Destination Module | Sample Response Time (ms) | Sample Response Time of Token Check (ms) |
|---|---|---|---|---|
| Initial request | 99 | |||
| Fetch user (making the request) | GET /users/{id} | mod-users | 6 | |
| Fetch permissions | GET /perms/users?query=userId=={id} | mod-permissions | 16 | |
| Generate downstream token | 12 | |||
| Fetch user (patron) by barcode | GET /users?query=barcode=={userBarcode} | mod-users | 13 | 86 |
| Fetch manual blocks | GET /manualblocks?query=userId=={userId} | mod-feesfines | 133 | 7 |
| Fetch automated blocks | GET /automated-patron-blocks/{userId} | mod-patron-blocks | 546* | 27 |
| Fetch item by barcode | GET /item-storage/items?query=barcode=={itemBarcode} | mod-inventory-storage | 163** | 10 |
| Fetch holdings | GET /holdings-storage/holdings/{id} | mod-inventory-storage | 57 | 9 |
| Fetch instance | GET /instance-storage/instances/{id} | mod-inventory-storage | 22 | 7 |
| Fetch location | GET /locations/{id} | mod-inventory-storage | 9 | 13 |
| Fetch library | GET /location/units/libraries/{id} | mod-inventory-storage | 10 | 7 |
| Fetch campus | GET /location/units/campuses/{id} | mod-inventory-storage | 10 | 7 |
| Fetch institution | GET /location/units/institutions/{id} | mod-inventory-storage | 11 | 7 |
| Fetch service point | GET /service-points/{id} | mod-inventory-storage | 9 | 8 |
| Fetch material type | GET /material-types/{id} | mod-inventory-storage | 8 | 7 |
| Fetch loan type | GET /loan-types/{id} | mod-inventory-storage | 22 | 8 |
| Fetch existing loans | GET /loan-storage/loans?query=status.name=="Open" and itemId=={itemId} | mod-circulation-storage | 9 | 17 |
| Fetch requests | GET /request-storage/requests?query=itemId=={itemId} and status==("Open - Not yet filled" or "Open - Awaiting pickup" or "Open - In transit" or "Open - Awaiting delivery") sortBy position/sort.ascending | mod-circulation-storage | 10 | 9 |
| Fetch circulation rules | GET /circulation/rules | mod-circulation-storage | 18 | 18 |
| Fetch loan policy | GET /loan-policy-storage/loan-policies/{id} | mod-circulation-storage | 10 | 8 |
| Fetch tenant locale | GET /configurations/entries?query=module=="ORG" and configName=="localeSettings" | mod-configuration | 16 | 10 |
| Fetch overdue fines policies | GET /overdue-fines-policies/{id} | mod-feesfines | 19 | 8 |
| Fetch lost item fees policies | GET /lost-item-fees-policies/{id} | mod-feesfines | 11 | 10 |
| Fetch opening days | GET /calendar/periods/7068e104-aa14-4f30-a8bf-71f71cc15e07/calculateopening?requestedDate={{dueDate}} | mod-calendar | 12 | 8 |
| Fetch user (patron) groups | GET /groups?query=id=={groupId} | mod-users | 17 | 7 |
| Update item status | PUT /item-storage/items/{id} | mod-inventory-storage | 194 | 13 |
| Create loan | POST /loan-storage/loan | mod-circulation-storage | 16 | 8 |
Update  patron action session | POST /patron-action-session-storage/patron-action-sessions | mod-circulation-storage | 10 | 7 |
| Fetch user | GET /users/{id} | mod-users | 6 | 15 |
| Fetch patron notice policy | GET /patron-notice-policy-storage/patron-notice-policies/1a821238-0cd9-48d9-a71a-057d33df0154 | mod-circulation-storage | 6 | 7 |
* The Vega team have already done some work to improve this
** The Core Platform team have already done work to improve this