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Posted to issues@trafficcontrol.apache.org by GitBox <gi...@apache.org> on 2020/04/13 20:03:08 UTC

[GitHub] [trafficcontrol] ocket8888 commented on a change in pull request #4628: Add ORT Rewrite Blueprint

ocket8888 commented on a change in pull request #4628: Add ORT Rewrite Blueprint
URL: https://github.com/apache/trafficcontrol/pull/4628#discussion_r407687987

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File path: blueprints/ort-rewrite-unix-style.md
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+
+# ORT Rewrite in UNIX Philosophy
+
+## Problem Description
+ORT is:
+- Difficult to maintain. Writing Perl is difficult, and reading it is even more difficult.
+- Dangerous to modify. Perl is not compiled, and even validity checks (`perl –c`) fail to verify dynamic runtime errors. This makes it very easy to introduce a bug in seldom-executed areas.
+- Untested. Perl ORT has no unit or integration tests.
+- Opaque. Nobody really knows everything it does, or when, or why.
+
+## Proposed Change
+
+ORT will be rewritten into a series of standalone executables, in the "UNIX Philosophy"
+
+> 1. Make each program do one thing well. To do a new job, build afresh rather than complicate old programs by adding new "features".
+> 2. Expect the output of every program to become the input to another, as yet unknown, program. Don't clutter output with extraneous information. Avoid stringently columnar or binary input formats. Don't insist on interactive input.
+
+- Each executable should do exactly 1 thing, and if a new "thing" becomes necessary, a new executable will be created.
+- The input and output of executables should be text which is easily parseable, so the executables can easily be pipelined (passing the output of one to the input of another), as well as easily read by humans and manipulated by standard Linux/POSIX tools.
+
+This makes ORT:
+- Easier to maintain. Each binary does one thing, is much smaller, and is more obvious. Presumably they’re also written in a language easier to read and write, such as Go.
+- Safer to modify. If each component is smaller, it’s more obvious what it does. We also presume the apps will be written with good development practices (such as modularization), with a language which verifies more at compile-time, and with tests.
+- Clear and easy for operators to understand what each app does. We assume clean interfaces, and good documentation (ideally in the app itself, via help flags, printing usage when no arguments are received, and/or man pages).
+
+#### Implementation
+
+The implementation should adhere to the "UNIX Philosophy," POSIX, Linux Standard Base (LSB), and GNU as much as possible.
+
+ORT will continue to consist of a single OS package (e.g. RPM), which installs all executables.
+
+ORT will require the following executables:
+- **Aggregator**. This is the “primary application” which will emulate the existing ORT script, and be called by CRON or operators to deploy all configs, as ORT does today. Note this is similar to how git works, and several other common Linux CLI utilities.
+ This app will have no logic itself, except to call the other executables.
+ - INPUT: configuration and specification to fetch and emplace config files.
+ - BEHAVIOR: fetches and places config files
+ - OUTPUT: success or failure message
+
+- **Traffic Ops Requestor**. This will fetch data needed from Traffic Ops, such as the Update Pending flag, packages, etc. This should never modify TO data, and should be guaranteed read-only. Any status modifications should go in the Traffic Ops Updater.
+ - INPUT: Traffic Ops URL and credentials, and data to fetch
+ - BEHAVIOR: Requests data from Traffic Ops
+ - OUTPUT: Traffic Ops data requested
+ - Format is probably multipart/mixed, but format may be different if a better format is determined. Ideal "UNIX Philosophy" format is line-delimited text, but the complexity may preclude that. The more complex and difficult to parse, the further from the "UNIX Philosophy." E.g. multipart/mixed is preferable to JSON.
+- **Config File Generator**. This will take TO data and produce config files.
+ - INPUT: Traffic Ops data, and config file(s) to generate or exclude (typically all, possibly “reval only” or other behaviors of ORT)
+ - BEHAVIOR: No side effects. Computationally: builds requested files.
+ - OUTPUT: Config files
+ - Format is probably multipart/mixed, but as above, may be different, multipart is preferable to JSON, etc.
+- **Config File Preprocessor**. Preprocesses generated config files, making post-generation modifications such as underscore directive replacements, and remap OVERRIDE replacements. TODO: determine if this should be rolled into the Config File Generator.
+ - INPUT: Config files, TO data/metadata.
+ - BEHAVIOR: No side effects; computationally: modifies files per rules and data.
+ - OUTPUT: modified config files.
+- **Server Config Readiness Verifier**. Verifies the operating system is ready and safe to apply the given config. Currently, this is just checking udev rules and verifying ATS block devices don’t have filessytems. But it may be more in the future. TODO: determine if necessary; should this even be ORT’s job? Will we ever need anything besides udev/50-ats.rules?
+ - INPUT: config files
+ - BEHAVIOR: No side effects; reads configs and inspects server state.
+ - OUTPUT: whether server is safe and ready to apply config files.
+- **ATS Plugin Readiness Verifier**. Verifies ATS has the necessary plugins for the config files.
+ - INPUT: config files
+ - BEHAVIOR: No side effects; reads configs and inspects server state.
+ - OUTPUT: whether ATS has all necessary plugins, or which files require which missing plugins.
+- **Diff Tool**. This will take two config files (presumably an existing and new file) and return their differences. TODO: determine if this is necessary; if configs are deterministic, can POSIX diff be used?
+ - INPUT: Two config files
+ - BEHAVIOR: No side effects. Computationally: diffs given files.
+ - OUTPUT: file diff. Ideally in a standard format.
+- **Backup Tool**. This will take a file and copy it to a backup location. TODO: determine if necessary; are standard POSIX cp/mv/etc enough?
+ - INPUT: config file to backup, and backup location. May be text of new file or path of existing file.
+ - BEHAVIOR: Save or copy given file to given backup location. Should be atomic.
+ - OUTPUT: success or failure message.
+- **Restart Determiner**. Takes the config to be applied (only the changed files, after diffing), encapsulates the logic of what changes require a reload or restart, and returns whether a restart, reload, or neither is required.
+ - INPUT: config files to be applied.
+ - BEHAVIOR: no side effects; computationally: inspects files and determines action.
+ - OUTPUT: whether ATS needs reloaded or restarted.
+- **Service Reloader**. Takes the name of the service (possibly only ATS) to reload or restart, and reloads or restarts as necessary. TODO determine if necessary; is this any logic beyond calling service restart and/or traffic_ctl?
+ - INPUT: service, whether to reload or restart
+ - BEHAVIOR: reloads or restarts the service
+ - OUTPUT: success or failure
+- **Traffic Ops Updater**. This will set the server’s update status in Traffic Ops. This should be only the Update and Reval Pending flags; ORT should never modify server configuration data, only ever server configuration status data.
+ - INPUT: Traffic Ops URL and credentials, and status to set
+ - BEHAVIOR: Makes a POST request to TO setting the status
+ - OUTPUT: success or failure message
+
+#### Features Omitted
+
+The following features of the current ORT are specifically not being implemented in the redesign:
+
+- **Chkconfig**. Chkconfig is not used by CentOS 7+; specifically, SystemD does not use it. It is misleading that ORT sets it today.
+- **Ntpd**. ORT currently has custom logic to restart ntpd if an ntpd.conf is changed. This should be managed by whatever system is managing the server (Ansible/Puppet/Manual/etc). Network time should not be the responsibility of Traffic Control or its config applicator.
+- **Interactive mode**. This mode is rarely possibly never used today. Further, by dividing ORT into UNIX-style apps for each function, an operator can easily see what results would be before running them.
+- **Revalidate Mode**. ORT is now fast enough to make a separate Revalidate unnecessary. It should always check and apply all files.
+- **Report Mode**. The Aggregator itself does not need a Report Mode. Operators wanting to see what would be changed can run the Traffic Ops Requestor, Config File Generator, Diff Tool, Restart Determiner, and Package Install Determiner.
+- **Package Installation**. ORT will cease to perform this. OS (RPM) package installation will no longer be done by Traffic Control, but rather by whoever or whatever is managing the machine and operating system (Ansible, Puppet, human system administrators, etc).
+ - Whatever is managing the other hundreds of packages on the operating system should also manage ATS and its plugins. ORT's job is to manage Traffic Control configuration data, not the operating system.
+
+#### Additional Utilities.
+
+Shell scripts which are “one-liners” pipelining common operations should be provided with the OS Package. TODO: add a list of scripts under Implementation heading.
+
+Additionally, a .pl script which emulates the existing ORT behavior will be provided in the old location, to preserve backwards-compatibility. This script should be very small, and essentially translate old calls and flags to the new Aggregator.
+
+#### Notes
+
+I started to list requirements for all apps, such as unit tests, integration tests, modular design, argument/manpage for usage info, etc. But then I realized I was just listing good design principles. So I decided to omit that.
+
+### Traffic Portal Impact
+None.
+
+### Traffic Ops Impact
+None.
+
+#### REST API Impact
+None.
+
+#### Client Impact
+None.
+
+#### Data Model / Database Impact
+None.
+
+### ORT Impact
+Completely rewrites ORT. Backward-compatibility for safe upgrades will be preserved.
+
+The Interface will be similar, but compatibility is not a goal. Further, LSB-compliant options and parameters is a goal, and will require incompatibility.
+
+A `traffic_ops_ort.pl` script will be provided, whose interface _does_ preserve backward compatibility, and calls the new ORT "aggregator". This will preserve existing CRON or other tools users are using with ORT, and make an upgrade not break a production system.
+
+### Traffic Monitor Impact
+None.
+
+### Traffic Router Impact
+None.
+
+### Traffic Stats Impact
+None.
+
+### Traffic Vault Impact
+None.
+
+### Documentation Impact
+Rewrite will provide MAN pages for ORT. ORT arguments are not currently documented, and that will not be changed, to avoid duplicate documentation. TC "read-the-docs" may include a small comment pointing users to the MAN page.

Review comment:
Sphinx has an output mode for manual pages. So the official docs for the utility could also be its man pages.

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