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This diff contains readability improvements for the step-by-step design document.

Co-authored-by: Simone Basso <bassosimone@gmail.com>
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docs/design/dd-003-step-by-step.md
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@ -1,16 +1,19 @@
# Step-by-step measurements
| | |
|:-------------|:-------------|
|--------------|------------------------------------------------|
| Author | [@bassosimone](https://github.com/bassosimone) |
| Last-Updated | 2022-06-13 |
| Last-Updated | 2022-06-29 |
| Reviewed-by | [@hellais](https://github.com/hellais) |
| Reviewed-by | [@DecFox](https://github.com/DecFox/) |
| Reviewed-by | [@ainghazal](https://github.com/ainghazal/) |
| Status | approved |
| Obsoletes | [dd-002-netx.md](dd-002-netx.md) |
*Abstract.* The original [netx design document](dd-002-netx.md) is now two
years old. Since we wrote such a document, we amended the overall design
## Abstract
The original [netx design document](dd-002-netx.md) is now two
years old. Since we wrote that document, we amended the overall design
several times. The four major design changes were:
1. saving rather than emitting
@ -24,50 +27,56 @@ pattern [ooni/probe-engine#522](https://github.com/ooni/probe-engine/pull/522);
4. measurex [ooni/probe-cli#528](https://github.com/ooni/probe-cli/pull/528).
In this (long) design document, we will revisit the original problem proposed by
[df-002-netx.md](df-002-netx.md), in light of what we changed and learned from the
changes we applied. We will highlight the significant pain points of the current
implementation, which are the following:
[dd-002-netx.md](dd-002-netx.md), in light of all the changes and lessons
learned since then. We will highlight the significant pain points of the
current implementation, which are the following:
1. that the measurement library API is significantly different from the Go stdlib
API, therefore violating this original `netx` design goal: that writing a new
experiment means using slightly different constructors that deviate from the standard
library only to meet specific measurement goals we have;
1. The measurement library API is significantly different from the Go stdlib
API. This violates one of the central design goals for `netx`: that writing a new
experiment would involve using constructors very similar to the standard
library. Such deviations were supposed to be made only to meet our specific
measurement goals;
2. that the decorator pattern leads to complexity in creating measurement types,
2. The decorator pattern has lead to complexity in creating measurement types,
which in turn seems to be the reason why the previous issue exists;
3. that the decorator pattern does not allow us to precisely collect all the
data that matters for events such as TCP connect and DNS round trips using
a custom transport, thus suggesting that we should revisit our choice of using
decorators and revert back to some form of _constructor based injection_ to
3. The decorator pattern does not allow us to precisely collect all the
data that matters for certain events (such as TCP connect and DNS round trips using
a custom transport). This suggests that we should revisit our choice of using
decorators, and revert back to some form of _constructor based injection_ to
inject a data type suitable for saving events.
In doing that, we will also propose an incremental plan for moving the tree
Finally, this document also proposes an incremental plan for moving the tree
forward from [the current state](https://github.com/ooni/probe-cli/tree/1685ef75b5a6a0025a1fd671625b27ee989ef111)
to a state where complexity is transferred from the measurement-support library to
the implementation of each individual network experiment.
to a state in which the complexity has been transferred from the
measurement-support library to the implementation of each individual network
experiment.
## Index
In [netxlite: the underlying library](#netxlite-the-underlying-network-library) we
describe the current design of the underlying network library.
There are four main sections in this document:
In [measurement tactics](#measurement-tactics) we provide an historical perspective
on the measurement tactics, we adopted or just tried.
[1. Netxlite: the underlying library](#1-netxlite-the-underlying-network-library)
describes the current design of the underlying network library.
The [step-by-step refactoring proposal](#step-by-step-refactoring-proposal)
contains the main contribution of this design document: a proposal to refactor
the existing codebase to address our current measurement-code problems.
[2. Measurement tactics](#2-measurement-tactics) gives an historical perspective
on different measurement tactics we adopted or tried in the past, and reflects
on their merits and downsides.
The [reviews](#reviews) section contains information about reviews.
[3. Step-by-step refactoring proposal](#3-step-by-step-refactoring-proposal)
contains the main contribution of this design document: a concrete proposal to
refactor the existing codebase to address our current measurement-code
problems.
## netxlite: the underlying network library
[4. Document reviews](#4-document-reviews) contains information about reviews of this document.
This section describes `netxlite`, the underlying network library, from a
## 1. netxlite: the underlying network library
This section describes `netxlite`, the underlying network library, from an
historical perspective. We explain our data collection needs, and what types
from the standard library we're using as patterns.
### Measurement Observations
### 1.1. Measurement Observations
Most OONI experiments need to observe and give meaning to these events:
@ -121,7 +130,7 @@ defines how we archive experiment results as a set of observations.
(Orthogonally, we may also want to improve the data format, but this is
not under discussion now.)
### Error Wrapping
### 1.2. Error Wrapping
The OONI data format also specifies [how we should represent
errors](https://github.com/ooni/spec/blob/master/data-formats/df-007-errors.md).
@ -140,7 +149,7 @@ DNS response messages or the syscall error returned by a Read call. By
adding this, we would give those who analyze the data information to
evaluate the correctness of a measurement.
### Go Stdlib
### 1.3 Go Stdlib
The Go standard library provides the following structs and interfaces
that we can use for measuring:
@ -235,7 +244,7 @@ Apart from the stdlib and quic-go, the only other significant network code
dependency is [miekg/dns](https://github.com/miekg/dns)
for custom DNS resolvers (e.g., DNS-over-HTTPS).
### Network Extensions
### 1.4. Network Extensions
A reasonable idea is to try to use types as close as possible to the
standard library. By following this strategy, we can compose our code
@ -281,12 +290,20 @@ netxlite as a dependency.
itself with measurements unlike [the original netx](df-002-netx.md), which contained
both basic networking wrappers and network measurement code.)
## Measurement Tactics
## 2. Measurement Tactics
Each subsection presents a different tactic for collecting measurement
observations.
observations, while reflecting on their pros and cons.
### Context-Based Tracing
We revisit four distinct tactics:
* [(1) Context-based tracing](#21-context-based-tracing),
* [(2) Decorator-based tracing](#22-decorator-based-tracing),
* [(3) Step-by-step measurements](#23-step-by-step-measurements), and
* [(4) Measurex: splitting DNSLookup and Endpoint Measurements](#24-measurex-splitting-dnslookup-and-endpoint-measurements).
### 2.1. Context-Based Tracing
This tactic is the first one we implemented. We call this approach
"tracing" because it produces a trace of the events, and it's
@ -304,7 +321,7 @@ the stdlib allows one to use the context to perform network tracing
we progressively abandoned `httptrace` as our tracing needs
become more complex than what `httptrace` could provide us with.
#### How this tactic feels like
#### How context-tracing feels like
I tried to adapt how this code would look if we used it now. As
[dd-002-netx.md](dd-002-netx.md) suggests, here I am trying to separate
@ -346,7 +363,7 @@ As you can see, I have marked with fire emojis where we
need to figure out what happened by reading the trace. We are going
to discuss this issue in the next section.
#### Issue #1: distance between collection and interpretation
#### Issue #1 with context tracing: distance between collection and interpretation
The nice part of this approach is that the network-interaction part of
the experiment is \~easy. The bad part is that we must figure out
@ -422,7 +439,7 @@ respect and make result-determining code more obvious and closer to the code
that performs the measurement. We will eventually come to fix this issue later
in this document. For now, let us continue to analyze this tactic.
#### Issue #2: the Context is magic and implicit
#### Issue #2 with context tracing: the Context is magic and implicit
Another pain point is that we're using the Context's magic. What happens
there feels more obscure than explicit initialization for performing
@ -444,7 +461,7 @@ dialer := saver.WrapQUICDialer(dialer)
conn, err := dialer.DialContext(ctx, /* ... */)
```
In the latter case, it's evident that we're *decorating* the original
In the later case, it's evident that we're *decorating* the original
dialer with an extended dialer that knows how to perform network
measurements. In the former case, it feels magic that we're setting some
value as an opaque any inside of the context, and there's a documented
@ -460,7 +477,7 @@ explicitly wrapping a type. I will discuss this topic when we
analyze the next tactic because the next tactic is all about reducing
the cognitive burden and avoiding the context.
#### Issue #3: we obtain a flat trace
#### Issue #3 with context tracing: we obtain a flat trace
The most straightforward implementation of this approach yields a flat trace. This
means that one needs to be careful to understand, say, which events are
@ -474,7 +491,7 @@ assign numbers to distinct HTTP round trips and DNS lookups so that
later it is possible to make sense of the trace. This was indeed the
approach we chose initially. However, this approach puts more pressure
on the context magic because it does not just suffice to wrap the
context once with WithValue, but you need to additionally wrap it when
context once with `WithValue`, but you need to additionally wrap it when
you descend into sub-operations. (Other approaches are possible, but I
will discuss this one because it looks conceptually cleaner to
create a new "node" in the trace like it was a DOM.)
@ -529,7 +546,7 @@ censorship. See our [DoT in Iran
research](https://ooni.org/post/2020-iran-dot/) for more
information.)
### Decoration-Based Tracing
### 2.2. Decorator-Based Tracing
In [probe-engine#359](https://github.com/ooni/probe-engine/issues/359), we
started planning refactoring of `netx` to solve the identified issues
@ -903,7 +920,7 @@ the `CNAME` and getaddrinfo's return code very easily):
}
```
#### Concluding remarks
#### Concluding remarks on decorator-based tracing
Historically, the decorator-based approach helped simplify
the codebase (probably because what we had previously was
@ -922,29 +939,29 @@ Whatever choice we make, it should probably include some form of
dependency injection for a trace that allows us to collect the events we
care about more precisely and with less effort.
### Step-by-Step Measurements
### 2.3. Step-by-step measurements
We had previous conversations around simplifying how we perform
measurements (e.g., I remember a conversation with
[Vinicius](https://github.com/fortuna), where he advocated
for decomposing measurements in simple operations, and he
rightfully pointed out that tracing is excellent for debugging
but complicating assigning meaning to measurements).
We've had many conversations about how to simplify the way we do measurements.
For instance, [Vinicius](https://github.com/fortuna) at some point advocated
for decomposing measurements in simple operations. He rightfully pointed out
that tracing is excellent for debugging, but it complicates to assign
meaning to each measurement.
We also mentioned that, in the codebase, we documented that `netx` was discouraged as an
approach for new experiments. The first chance to try a different tactic
was the development of the `websteps` prototype. We tried to implement
step-by-step measurements, which, in its most radical form, calls for
performing each relevant step in isolation, immediately saving a
small trace and interpreting it before moving on to the next step
(unless there's an error, in which case you typically stop).
We had documented in the codebase that `netx` was discouraged as an approach
for new experiments. We got the first chance to try a
different tactic while developing the `websteps` prototype.
In `websteps`, we tried to implement step-by-step measurements: in its most
radical form, this calls for performing each relevant step in isolation,
immediately saving a small trace and interpreting it before moving on to the
next step (unless there's an error, in which case you typically stop).
Looking back at the Go stdlib API, the main blocker to implementing
this tactic is how to reconcile it with HTTP transports, which expects to
dial and control their own connections. Luckily,
[Kathrin](https://github.com/kelmenhorst)
[implemented](https://github.com/ooni/probe-cli/pull/432) the
following trick that solved such an issue:
following trick that allows us to solve this issue:
```Go
// NewSingleUseDialer returns a "single use" dialer. The first
@ -978,8 +995,8 @@ func (s *dialerSingleUse) DialContext(ctx context.Context, network string, addr
With a "single-use" dialer, we provide an HTTPTransport with a fake
dialer that provides a previously-established
connection to the transport itself. The following snippet shows
code from my first naive attempt at writing code using this tactic,
where originally identified pain points have been emphasized:
code from my first naive attempt at writing code using this approach. The pain
points we had originally identified have been emphasized:
```Go
dialer := netxlite.NewDialerWithoutResolver()
@ -1015,31 +1032,31 @@ if err != nil {
```
Let's discuss the good parts before moving on to the bad parts. It is
dead obvious which operation failed and why, and we know what went
wrong and can analyze the observations immediately.
dead obvious which operation failed and why, and **we know what went
wrong and can analyze the observations immediately**.
Additionally, if you ask someone who knows the standard library to write
an experiment that provides information about TCP connect, TLS handshake, and
HTTP round trip using `netxlite`, they would probably write something
similar to the above code.
#### Issue #1: no persistent connections
#### Issue #1 with step-by-step approach: no persistent connections
Without adding extra complexity, we lose the possibility of using
persistent connections. This may not be a huge problem except for
Without adding extra complexity, we lose the possibility to use
persistent connections. This may not be a huge problem, except for
redirects. Each redirect will require us to set up a new connection,
even though an ordinary http.Transport would probably have reused an
even though an ordinary `http.Transport` would probably have reused an
existing one.
Because we're measuring censorship, I would argue it's OK to not reuse
connections. Sure, the measurement could be slower, but we'd get
connections. Sure, the measurement can be slower, but we'll also get
more data points on TCP/IP and TLS blocking.
#### Issue #2: requires manual handling of redirects
#### Issue #2 with step-by-step approach: requires manual handling of redirects
Because we cannot reuse connections easily, we cannot rely on
&http.Client{} to perform redirections automatically for us. This is why
websteps implements HTTP redirection manually.
`&http.Client{}` to perform redirections automatically for us. This is why
`websteps` implements HTTP redirection manually.
While it may seem that discussing redirects is out of scope,
historically I had been reluctant to switch to a step-by-step model
@ -1075,7 +1092,7 @@ Because this problem of redirection is fundamental to many experiments
(not only webconnectivity but also, e.g., whatsapp), any step-by-step
approach library needs this functionality.
#### Issue #3: DRY pressure
#### Issue #3 with step-by-step approach: DRY pressure
Before, I said that saving traces seems complicated. That is not entirely
true. Depending on the extent to which we are willing to suffer the pain of
@ -1195,7 +1212,7 @@ some network operation. If the API is doing too much, I
might not have the ability to hook me into it and run the follow-up
experiment right after the operation I needed to do.
#### Concluding remarks
#### Concluding remarks on step-by-step measurements
If we have a way to collect observations,
this approach certainly has the advantage of having some
@ -1215,13 +1232,16 @@ probably want to split DNS and other operations to get a chance to
test all (or many) of the available IP addresses and use tracing within DNS and
other operations.
### DNSLookup and Endpoint Measurements
### 2.4. measurex: splitting DNSLookup and Endpoint Measurements
This approach is currently implemented as measurex. A DNSLookup
measurement should be an obvious concept, while it's probably less
clear what's an endpoint measurement. So, let's clarify.
This fourth and last approach of the ones we'll discuss is currently
implemented in `measurex`.
An endpoint measurement is one of the following operations:
A **DNSLookup measurement** is perhaps an obvious concept, but an **endpoint
measurement** is probably not obvious. So, let's first clarify the
terminology:
We define an **endpoint measurement** as one of the following operations:
1. given a TCP endpoint (IP address and port), TCP connect to it;
@ -1249,12 +1269,12 @@ addresses, so that pattern was already in OONI somehow (at least for the
most important experiment measuring web censorship.)
Another interesting observation about the above set of operations is
that each of them could fail exactly once. The DNSLookup could fail or
that each of them could fail **exactly once**. The DNSLookup could fail or
yield addresses, and TCP connect could fail or succeed. In the TCP connect
plus TLS handshake case, you stop there if you fail the TCP connect. And
so on.
Because of this reasoning, one could say that the measurex tactic
Because of this reasoning, one could say that the `measurex` tactic
is equivalent to the previous one in relatively easily identifying
the failed operation and filling the measurement. That seems to be an argument
for having a library containing code to simplify measurements.
@ -1263,15 +1283,17 @@ an entirely new API*. After careful consideration, it seems preferable to
select an API that is closer to what a typical Go programmer would
expect.
## Step-by-step refactoring proposal
## 3. Step-by-step refactoring proposal
Finally, all the discussion is in place to get to a concrete proposal.
I tried to reimplement the telegram experiment using a pure step-by-step approach
([here's the
gist](https://gist.github.com/bassosimone/f6e680d35805174d1f150bc15ef754af)).
It looks fine, but one ends up writing a support library such as measurex. Yet,
It looks fine, but one ends up writing a support library such as `measurex`. Yet,
as noted above, the API exposed by such a measurement
library matters, and an API familiar to Go developers seems preferable
to the API implemented by measurex.
to the API implemented by `measurex`.
There are two key insights I derived from my telegram PoC.
@ -1415,7 +1437,7 @@ the PoC is that *traces are numbered*. This is not what happens
currently in OONI Probe, but it is *beneficial*. By numbering
traces, we can quite easily tell which existing event belongs to which
specific submeasurement. (If there's no need to number traces, we can
just set a zero index to all the traces we collect, *e passa la paura*.)
just set a zero index to all the traces we collect, *[e passa la paura](https://context.reverso.net/traduzione/italiano-inglese/passa+la+paura)*.)
One minor aspect to keep in mind in this design is that we need to
communicate to developers that the *trace* will cause the body snapshot
@ -1423,14 +1445,14 @@ to be read as part of the round trip. This fact occurs because OONI's
definition of a request-response transaction includes the response body
(or a snapshot) while Go does not include a body in http.Response
but allows for streaming the body on demand. Because reading all the
body with netxlite.ReadAllContext without any limit bound is unsafe (as
body with `netxlite.ReadAllContext` without any limit bound is unsafe (as
it could consume lots of RAM, and we're not always running on systems
with lots of RAM), the example was already limiting the response
body length before we introduced data collection. Yet, with the introduction of
data collection, the explicit netxlite.ReadAllContext is now reading
data collection, the explicit `netxlite.ReadAllContext` is now reading
from memory rather than from the network because the body snapshot has
already been read. So, we need to ensure that developers know that
netxlite.ReadAllContext cannot be used to measure/estimate the
`netxlite.ReadAllContext` cannot be used to measure/estimate the
download speed. (In such a case, one would need to either use a
different transport or not collect any snapshot and then read
the whole body directly from the network--so perhaps we
@ -1453,16 +1475,16 @@ will always be some form of *limited* step-by-step, where we will always
split DNS lookup and endpoint measurements as we already do in
dnscheck to ensure we measure \~all IP addresses.
Compared to measurex, I think step-by-step is \~better because it does
not require anyone to learn more beyond how to use netxlite instead of
the standard library. (BTW, we cannot really get rid of netxlite because
Compared to `measurex`, I think step-by-step is \~better because it does
not require anyone to learn more beyond how to use `netxlite` instead of
the standard library. (BTW, we cannot really get rid of `netxlite` because
we have measurement requirements that call for wrapping and extending
the standard library or to provide enhancements beyond the stdlib
functionality.)
Regarding the way to implement tracing, from the above discussion, it is
clear that we should move away from the wrapping approach because it
does not allow us to correctly collect specific events. (To be fair, it
clear that **we should move away from the wrapping approach because it
does not allow us to correctly collect specific events**. (To be fair, it
could allow us to do that, but it would entail
significant wrapping efforts.) I would therefore recommend rewriting
tracing to use the context (ugh!) but to wrap this implementation inside
@ -1580,7 +1602,7 @@ can stick with the current model where we have possibly unbounded
traces. (This is just an implementation detail that does not matter much
regarding the overall design.)
### Smooth transition
### 3.1. Smooth transition
We should do incremental refactoring. We should create a few issues
describing these design aspects and summarize what would be the way
@ -1600,7 +1622,7 @@ earlier with a simplified tree with less measurement-supporting
libraries. It also seems that dash, hhfm, and hirl can be migrated quite
easily away from netx and urlgetter.
### Netxlite scope change
### 3.2. Netxlite scope change
If we move forward with this plan, we will slightly change the scope of
netxlite to include lightweight support for collecting traces. We
@ -1613,7 +1635,7 @@ policy for saving measurements by implementing model.Trace properly. So,
we should also amend the documentation of netxlite to
explicitly mention support for tracing as a new concern.
### Cleanups
### 3.3. Cleanups
If we implement this step-by-step change, we no longer need a "flat" data
format. We use the flat data format for processing the results
@ -1629,7 +1651,7 @@ concern is with the error wrapping, which probably should be in the same
place where we are using the context to inject a trace to ensure
that error wrapping and tracing happen together.)
## Reviews
## 4. Document Reviews
### 2022-06-09 - Review: Arturo

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internal/engine/experiment/README.md
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@ -1,4 +1,4 @@
# Directory github.com/ooni/probe-engine/experiment
# Directory github.com/ooni/probe-cli/internal/engine/experiment
This directory contains the implementation of all the supported
experiments, one for each directory. The [OONI spec repository

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internal/model/README.md
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@ -1,4 +1,4 @@
# Package github.com/ooni/probe-engine/model
# Package github.com/ooni/probe-cli/internal/model
Shared data structures and interfaces. We include in this
package the most fundamental types. Use `go doc` to get

2
internal/model/measurement.go
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@ -57,7 +57,7 @@ type Measurement struct {
// MeasurementStartTimeSaved is the moment in time when we
// started the measurement. This is not included into the JSON
// and is only used within probe-engine as a "zero" time.
// and is only used within the ./internal pkg as a "zero" time.
MeasurementStartTimeSaved time.Time `json:"-"`
// Options contains command line options