Back to blog
Research

APIs Explained: What Is an API and How It Works

A practical guide to what an API is, how APIs work, types and use cases, and a checklist to evaluate APIs for projects, products, and AI integrations.
Token Metrics Team
5
Want Smarter Crypto Picks—Free?
See unbiased Token Metrics Ratings for BTC, ETH, and top alts.
Thank you! Your submission has been received!
Oops! Something went wrong while submitting the form.
 No credit card | 1-click unsubscribe

APIs (application programming interfaces) are the invisible connectors that let software systems talk to each other. Whether you open a weather app, sign in with a social account, or call a machine-learning model, an API is usually orchestrating the data exchange behind the scenes. This guide explains what an API is, how APIs work, common types and use cases, and practical frameworks to evaluate or integrate APIs into projects.

What is an API? Definition & core concepts

An API is a set of rules, protocols, and tools that defines how two software components communicate. At its simplest, an API specifies the inputs a system accepts, the outputs it returns, and the behavior in between. APIs abstract internal implementation details so developers can reuse capabilities without understanding the underlying codebase.

Key concepts:

  • Endpoints: Network-accessible URLs or methods where requests are sent.
  • Requests & responses: Structured messages (often JSON or XML) sent by a client and returned by a server.
  • Authentication: Mechanisms (API keys, OAuth, tokens) that control who can use the API.
  • Rate limits: Constraints on how often the API can be called.

How APIs work: a technical overview

Most modern APIs use HTTP as the transport protocol and follow architectural styles such as REST or GraphQL. A typical interaction looks like this:

  1. Client constructs a request (method, endpoint, headers, payload).
  2. Request is routed over the network to the API server.
  3. Server authenticates and authorizes the request.
  4. Server processes the request, possibly calling internal services or databases.
  5. Server returns a structured response with status codes and data.

APIs also expose documentation and machine-readable specifications (OpenAPI/Swagger, RAML) that describe available endpoints, parameters, data models, and expected responses. Tools can generate client libraries and interactive docs from these specs, accelerating integration.

Types of APIs and common use cases

APIs serve different purposes depending on design and context:

  • Web APIs (REST/HTTP): Most common for web and mobile backends. Use stateless requests, JSON payloads, and standard HTTP verbs.
  • GraphQL APIs: Allow clients to request precisely the fields they need, reducing over-fetching.
  • RPC and gRPC: High-performance, typed remote procedure calls used in microservices and internal infrastructure.
  • SDKs and libraries: Language-specific wrappers around raw APIs to simplify usage.
  • Domain-specific APIs: Payment APIs, mapping APIs, social login APIs, and crypto APIs that expose blockchain data, wallet operations, and on-chain analytics.

Use cases span the product lifecycle: integrating third-party services, composing microservices, extending platforms, or enabling AI models to fetch and write data programmatically.

Evaluating and integrating APIs: a practical framework

When selecting or integrating an API, apply a simple checklist to reduce technical risk and operational friction:

  • Specification quality: Is there an OpenAPI spec, clear examples, and machine-readable docs?
  • Authentication: What auth flows are supported and do they meet your security model?
  • Rate limits & quotas: Do limits match your usage profile? Are paid tiers available for scale?
  • Error handling: Are error codes consistent and documented to support robust client logic?
  • Latency & reliability: Benchmark typical response times and uptime SLAs for production readiness.
  • Data semantics & provenance: For analytics or financial data, understand update frequency, normalization, and source trustworthiness.

Operationally, start with a sandbox key and integrate incrementally: mock responses in early stages, implement retry/backoff and circuit breakers, and monitor usage and costs in production.

Build Smarter Crypto Apps & AI Agents with Token Metrics

Token Metrics provides real-time prices, trading signals, and on-chain insights all from one powerful API. Grab a Free API Key

FAQ: Common questions about APIs

What is the difference between REST and GraphQL?

REST organizes resources as endpoints and often returns fixed data shapes per endpoint. GraphQL exposes a single endpoint where clients request the exact fields they need. REST is simple and cache-friendly; GraphQL reduces over-fetching but can require more server-side control and caching strategies.

How do API keys and OAuth differ?

API keys are simple tokens issued to identify a client and are easy to use for server-to-server interactions. OAuth provides delegated access where a user can authorize a third-party app to act on their behalf without sharing credentials; it's essential for user-consent flows.

Are there standards for API documentation?

Yes. OpenAPI (formerly Swagger) is widely used for REST APIs and supports automated client generation and interactive documentation. GraphQL has its own schema specification and introspection capabilities. Adopting standards improves developer experience significantly.

What security considerations matter most for APIs?

Common practices include strong authentication, TLS encryption, input validation, explicit authorization, rate limiting, and logging. For sensitive data, consider data minimization, field-level encryption, and strict access controls.

How can AI models use APIs?

AI models can call APIs to fetch external context, enrich inputs, or persist outputs. Examples include retrieving live market data, fetching user profiles, or invoking specialized ML inference services. Manage latency, cost, and error handling when chaining many external calls in a pipeline.

Disclaimer

This article is for educational and informational purposes only. It does not constitute professional, legal, or financial advice. Evaluate any API, provider, or integration according to your own technical, legal, and security requirements before use.

Build Smarter Crypto Apps &
AI Agents in Minutes, Not Months
Real-time prices, trading signals, and on-chain insights all from one powerful API.
Grab a Free API Key
About Token Metrics
Token Metrics: AI-powered crypto research and ratings platform. We help investors make smarter decisions with unbiased Token Metrics Ratings, on-chain analytics, and editor-curated “Top 10” guides. Our platform distills thousands of data points into clear scores, trends, and alerts you can act on.
30 Employees
analysts, data scientists, and crypto engineers
Daily Briefings
concise market insights and “Top Picks”
Transparent & Compliant
Sponsored ≠ Ratings; research remains independent
Want Smarter Crypto Picks—Free?
See unbiased Token Metrics Ratings for BTC, ETH, and top alts.
Thank you! Your submission has been received!
Oops! Something went wrong while submitting the form.
 No credit card | 1-click unsubscribe
Token Metrics Team
Token Metrics Team

Recent Posts

Research

Top 100 Crypto Index: What It Is, How It’s Built, and Who It’s For (2025)

Token Metrics Team
6

If you’ve tried to “own the market” in crypto, you’ve felt the pain: chasing listings, juggling wallets, and missing rebalances while prices move. A top 100 crypto index aims to fix that—giving you broad exposure when the market is bullish and standing down when it’s not. Token Metrics Global 100 is our rules-based version of that idea: it holds the top-100 by market cap in bull regimes, moves to stablecoins in bear regimes, and rebalances weekly. You can see every rule, every holding, and every rebalance—then buy the index in ~90 seconds with an embedded on-chain flow.‍ → Join the waitlist to be first to trade TM Global 100.

Why a “Top 100 Crypto Index” Matters in October 2025

The market keeps cycling. New leaders emerge quickly. A “set-and-forget” bag can fall behind, while manual baskets burn hours and rack up slippage. Search interest for crypto index, regime switching, and weekly rebalancing keeps growing because people want a simple, disciplined core that adapts.

A top 100 crypto index is a rules-based basket that tracks the largest 100 crypto assets by market cap, typically rebalanced on a schedule to keep weights aligned with the market. In 2025, that alone isn’t enough. You also need discipline for downtrends. Token Metrics adds a regime-switching layer to move to stablecoins during bear phases—so you can participate in upside and sit out major drawdowns with a consistent, rules-based approach.

How the TM Global 100 Index Works (Plain English)

Regime switching:

  • Bullish: The index holds the top-100 assets by market cap.
  • Bearish: The index exits positions and moves fully to stablecoins until a bullish re-entry signal.

Weekly rebalancing:

  • Every week, the composition and weights update to reflect current market-cap rankings. No manual list maintenance. No “oops, I missed the new entrant.”

Transparency:

  • Strategy modal explains selection criteria and regime logic.
  • Gauge → Treemap → Transactions Log shows the signal, the real-time holdings view, and every rebalance/regime switch.

You’ll always see what you own, how it changed, and why.

What you’ll see on launch

  • Price tile, 100 tokens, “rebalances weekly,” and one-click Buy.
  • Gauge to visualize the market signal.
  • Holdings Treemap and Table to inspect exposure.
  • Transactions Log to review every rebalance.
  • See the strategy and rules.

Benefits at a Glance (Why This Beats DIY)

  • Time saved: Skip hours of asset chasing and manual spreadsheets; rebalances happen automatically.
  • Lower execution drag: One index buy can reduce slippage vs. piecing together 20–50 small orders across chains.
  • Never miss a rebalance: Weekly updates and on/off risk switches run by rules, not vibes.
  • Rules-based switching: A clear trigger defines when to sit in stablecoins—no second-guessing.
  • Full visibility: The gauge, treemap, table, and log make the process auditable at a glance.
  • Operational simplicity: An embedded wallet, 90-second buy flow, fee and slippage estimates upfront.

Step-by-Step: How to Get Early Access (Waitlist)

  1. Open the Indices hub and tap TM Global 100.
  2. Join the waitlist with your email—this flags you for day-one access.
  3. (Optional) Connect your wallet so you’re ready for the embedded checkout.
  4. Launch day: You’ll get an email and in-app prompt when trading opens.
  5. Buy in ~90 seconds: Connect, review fees/slippage/estimated value, confirm.
  6. Track positions: See your holdings, rebalances, and P&L in My Indices.
  7. Repeat or add funds: Rebalancing is handled weekly; you can add or sell anytime.

→ Join the waitlist to be first to trade TM Global 100.

Decision Guide: Is This Right for You?

  • Hands-Off Allocator: Want broad market exposure without managing coin lists? Consider it.
  • Active Trader: Want a disciplined core you don’t have to watch while you chase setups? Consider it.
  • TM Member (Research-Heavy): Prefer to keep your picks, but want a market base layer? Consider it.
  • New to Crypto: Need transparency + clear rules? Consider it, with a small test first.
  • Hyper-Niche Maxi: If you only want 1–2 coins, an index may be too broad.
  • Short-Term Scalper: You may still benefit from a core allocation, but active trading stays your main driver.
  • Tax-/Jurisdiction-Sensitive Users: Check your local rules before investing.
  • Institutional Explorers: Looking for transparent rules, logs, and weekly governance? Worth evaluating.

FAQs

What is a top 100 crypto index?

A rules-based basket tracking the largest 100 assets by market cap, typically with scheduled rebalancing. Token Metrics Global 100 adds regime switching to stablecoins during bear markets.

How often does the index rebalance?

Weekly. In addition, if the market signal flips, the entire portfolio may switch between tokens ↔ stablecoins outside the weekly cycle.

What triggers the move to stablecoins?

A proprietary market-regime signal. When it’s bearish, the index exits tokens to stablecoins and waits for a bullish re-entry signal.

Can I fund with USDC or fiat?

On launch, funding options surface based on your connected wallet and supported chains. USDC payouts are supported when selling.

Is the wallet custodial?

The embedded wallet is self-custodial—you control your funds.

How are fees shown?

Before you confirm a buy, you’ll see estimated gas, platform fee, max slippage, and minimum expected value—all up front.

How do I join the waitlist?

Go to the TM Global 100 page or the Indices hub and click Join Waitlist. You’ll get notified at launch with simple steps to buy.

Security, Risk & Transparency

  • Self-custody: Embedded, self-custodial smart wallet; you control keys.
  • 2FA & device checks: Standard authentication best practices.
  • Fee/slippage transparency: All estimates are shown pre-trade; you confirm with eyes open.
  • On-chain visibility: Holdings, rebalances, and regime switches appear in the Transactions Log.
  • Rule constraints: Signals can be wrong; spreads and volatility can impact outcomes.
  • Regional considerations: Availability and tax treatment vary by jurisdiction.

Crypto is volatile and can lose value. Past performance is not indicative of future results. This article is for research/education, not financial advice.

A top 100 crypto index is the simplest path to broad market exposure—if it’s built with discipline. Token Metrics Global 100 combines transparent rules, weekly rebalancing, and a regime switch to stablecoins, so you can focus on your strategy while the core maintains itself. Now’s the time to claim early access.‍ → Join the waitlist to be first to trade TM Global 100.

Research

The Case for Rules-Based Crypto Indexing After a Volatile Cycle (2025)

Token Metrics Team
5

After a whipsaw year, many investors are asking how to stay exposed to crypto’s upside without riding every drawdown. Rules-based crypto indexing is a simple, disciplined answer: follow a transparent set of rules rather than gut feelings. The Token Metrics Global 100 puts this into practice—own the top-100 in bullish regimes, rotate to stablecoins in bearish regimes, and rebalance weekly. On top of that, you can see what you own in real time with a Holdings Treemap, Table, and Transactions Log. Less second-guessing, more process.

→ Join the waitlist to be first to trade TM Global 100.

Why Rules-Based Crypto Indexing Matters in October 2025

In a volatile cycle, emotion creeps in: chasing winners late, cutting losers early, or missing re-entry after fear. Rules-based crypto indexing applies consistent criteria—constituent selection, weighting, and rebalancing—so you don’t have to improvise in stress.

For readers comparing crypto index options, think of it as a codified playbook. A rules-based crypto index is a methodology-driven basket that follows predefined signals (e.g., market regime) and maintenance schedules (e.g., weekly rebalancing), aiming for repeatable behavior across cycles.

Rules-based crypto indexing is a systematic approach that tracks a defined universe (e.g., top-100 by market cap) and maintains it on a fixed cadence, with explicit rules for when to hold tokens and when to de-risk into stablecoins.

How the TM Global 100 Index Works

  • Regime switching: When the market signal is bullish, the index holds the top 100 assets by market cap; when bearish, it moves to stablecoins until conditions improve.
  • Weekly rebalancing: Constituents and weights update weekly to reflect the latest market-cap rankings—capturing leadership changes without manual effort.
  • Transparency: A Strategy modal and Gauge → Treemap → Transactions Log show the signal, current mix, and every change recorded.

What you’ll see on launch: Price tile, “tokens: 100,” “rebalances weekly,” and a fast ~90-second Buy flow with fee/slippage previews.

See the strategy and rules. (TM Global 100 strategy)

Benefits at a Glance (Why This Beats DIY)

  • Time & operational drag: Skip juggling 20–100 tickers, wallets, and venues.
  • Execution quality: A single indexed flow can help reduce piecemeal slippage and duplicated fees.
  • No missed rotations: Weekly rebalancing and regime switching reduce the cost of being late to trends—or late to de-risk.
  • Always-on visibility: Holdings treemap + table + transactions log remove the black box.
  • Behavioral edge: Clear rules can limit panic sells and FOMO buys during turbulence.
  • Portfolio role: A disciplined core that you can complement with selective satellites.

Step-by-Step: How to Get Early Access (Waitlist)

  1. Open the Token Metrics Indices hub and select TM Global 100.
  2. Click Join Waitlist and enter your email for launch-day access.
  3. (Optional) Connect your wallet so you’re ready to fund.
  4. On launch, review the Gauge → Treemap → Transactions to confirm the current mix.
  5. Tap Buy Index, review fees/slippage, and confirm (about 90 seconds end-to-end).
  6. Track your position and every weekly rebalance in My Indices and the Transactions Log.

→ Join the waitlist to be first to trade TM Global 100.

Decision Guide: Is This Right for You?

  • Hands-Off Allocator: Want broad market beta with an explicit de-risking rule. Consider if you resist micromanaging.
  • Active Trader: Prefer a disciplined core that moves to stablecoins in bears while you express edge with satellites.
  • Long-Term Believer: Seek systematic participation in leadership changes via weekly rebalancing.
  • Transparency-First User: Require auditable holdings and a transactions log—no black boxes.
  • Tax/Compliance Conscious: Prefer consolidated rebalances over many ad hoc trades.
  • TM Research Follower: Want to pair Token Metrics insights with a rules-based execution layer.
  • New to Crypto Baskets: Want to avoid building and maintaining a DIY index.

FAQs

What is a rules-based crypto index?

A methodology-driven basket that follows predefined rules for asset selection, weighting, and maintenance. In TM Global 100, that means top-100 exposure in bullish regimes and stablecoins in bearish regimes, with weekly rebalancing and full transparency.

How often does the index rebalance?

Weekly. This cadence refreshes constituents and weights to align with current market-cap rankings; separate regime switches can move between tokens and stablecoins.

What triggers the move to stablecoins?

A documented market signal. When it turns bearish, the index exits to stablecoins; when bullish resumes, it re-enters the top-100 basket.

Can I fund with USDC or fiat?

Funding options will surface based on your connected wallet and supported rails. USDC settlement on sells is supported; fiat on-ramps may be added over time.

Is the wallet custodial?

No. The embedded wallet is self-custodial—you control your keys and assets.

How are fees shown?

Before confirming a trade, you’ll see estimated gas, platform fee, max slippage, and min expected value—so you can proceed with clarity.

How do I join the waitlist?

Go to the Indices hub, open TM Global 100, and enter your email. You’ll receive a launch-day link to buy.

Security, Risk & Transparency

  • Self-custody by default: You control your wallet.
  • Defense-in-depth: 2FA/account security features and explicit transaction prompts.
  • Clear economics: Fee and slippage previews before you confirm.
  • Auditability: Holdings treemap + table + transactions log document every change.
  • Methodology limits: Regime logic may not capture every market nuance; weekly cadence can differ from intraday moves.
  • Regional availability: On-ramps and features can vary by jurisdiction.

Crypto is volatile and can lose value. Past performance is not indicative of future results. This article is for research/education, not financial advice.

Research

The Case for Rules-Based Crypto Indexing After a Volatile Cycle (2025)

Token Metrics Team
5

After a whipsaw year, many investors are asking how to stay exposed to crypto’s upside without riding every drawdown. Rules-based crypto indexing is a simple, disciplined answer: follow a transparent set of rules rather than gut feelings. The Token Metrics Global 100 puts this into practice—own the top-100 in bullish regimes, rotate to stablecoins in bearish regimes, and rebalance weekly. On top of that, you can see what you own in real time with a Holdings Treemap, Table, and Transactions Log. Less second-guessing, more process.→ Join the waitlist to be first to trade TM Global 100.

Why Rules-Based Crypto Indexing Matters in October 2025

In a volatile cycle, emotion creeps in: chasing winners late, cutting losers early, or missing re-entry after fear. Rules-based crypto indexing applies consistent criteria—constituent selection, weighting, and rebalancing—so you don’t have to improvise in stress.

For readers comparing crypto index options, think of it as a codified playbook. A rules-based crypto index is a methodology-driven basket that follows predefined signals (e.g., market regime) and maintenance schedules (e.g., weekly rebalancing), aiming for repeatable behavior across cycles.

Rules-based crypto indexing is a systematic approach that tracks a defined universe (e.g., top-100 by market cap) and maintains it on a fixed cadence, with explicit rules for when to hold tokens and when to de-risk into stablecoins.

How the TM Global 100 Index Works (Plain English)

  • Regime switching: When the market signal is bullish, the index holds the top 100 assets by market cap; when bearish, it moves to stablecoins until conditions improve.
  • Weekly rebalancing: Constituents and weights update weekly to reflect the latest market-cap rankings—capturing leadership changes without manual effort.
  • Transparency: A Strategy modal and Gauge → Treemap → Transactions Log show the signal, current mix, and every change recorded.

What you’ll see on launch: Price tile, “tokens: 100,” “rebalances weekly,” and a fast ~90-second Buy flow with fee/slippage previews.

See the strategy and rules. (TM Global 100 strategy)

Benefits at a Glance (Why This Beats DIY)

  • Time & operational drag: Skip juggling 20–100 tickers, wallets, and venues.
  • Execution quality: A single indexed flow can help reduce piecemeal slippage and duplicated fees.
  • No missed rotations: Weekly rebalancing and regime switching reduce the cost of being late to trends—or late to de-risk.
  • Always-on visibility: Holdings treemap + table + transactions log remove the black box.
  • Behavioral edge: Clear rules can limit panic sells and FOMO buys during turbulence.
  • Portfolio role: A disciplined core that you can complement with selective satellites.

Step-by-Step: How to Get Early Access (Waitlist)

  1. Open the Token Metrics Indices hub and select TM Global 100.
  2. Click Join Waitlist and enter your email for launch-day access.
  3. (Optional) Connect your wallet so you’re ready to fund.
  4. On launch, review the Gauge → Treemap → Transactions to confirm the current mix.
  5. Tap Buy Index, review fees/slippage, and confirm (about 90 seconds end-to-end).
  6. Track your position and every weekly rebalance in My Indices and the Transactions Log.

→ Join the waitlist to be first to trade TM Global 100.

Decision Guide: Is This Right for You?

  • Hands-Off Allocator: Want broad market beta with an explicit de-risking rule. Consider if you resist micromanaging.
  • Active Trader: Prefer a disciplined core that moves to stablecoins in bears while you express edge with satellites.
  • Long-Term Believer: Seek systematic participation in leadership changes via weekly rebalancing.
  • Transparency-First User: Require auditable holdings and a transactions log—no black boxes.
  • Tax/Compliance Conscious: Prefer consolidated rebalances over many ad hoc trades.
  • TM Research Follower: Want to pair TM insights with a rules-based execution layer.
  • New to Crypto Baskets: Want to avoid building and maintaining a DIY index.

FAQs

What is a rules-based crypto index?

A methodology-driven basket that follows predefined rules for asset selection, weighting, and maintenance. In TM Global 100, that means top-100 exposure in bullish regimes and stablecoins in bearish regimes, with weekly rebalancing and full transparency.

How often does the index rebalance?

Weekly. This cadence refreshes constituents and weights to align with current market-cap rankings; separate regime switches can move between tokens and stablecoins.

What triggers the move to stablecoins?

A documented market signal. When it turns bearish, the index exits to stablecoins; when bullish resumes, it re-enters the top-100 basket.

Can I fund with USDC or fiat?

Funding options will surface based on your connected wallet and supported rails. USDC settlement on sells is supported; fiat on-ramps may be added over time.

Is the wallet custodial?

No. The embedded wallet is self-custodial—you control your keys and assets.

How are fees shown?

Before confirming a trade, you’ll see estimated gas, platform fee, max slippage, and min expected value—so you can proceed with clarity.

How do I join the waitlist?

Go to the Indices hub, open TM Global 100, and enter your email. You’ll receive a launch-day link to buy.

Security, Risk & Transparency

  • Self-custody by default: You control your wallet.
  • Defense-in-depth: 2FA/account security features and explicit transaction prompts.
  • Clear economics: Fee and slippage previews before you confirm.
  • Auditability: Holdings treemap + table + transactions log document every change.
  • Methodology limits: Regime logic may not capture every market nuance; weekly cadence can differ from intraday moves.
  • Regional availability: On-ramps and features can vary by jurisdiction.

Crypto is volatile and can lose value. Past performance is not indicative of future results. This article is for research/education, not financial advice.

After a volatile cycle, the edge is process. Token Metrics Global 100 combines rules-based crypto indexing, weekly rebalancing, and full transparency so you can participate in upside and step aside during bears—without running your own spreadsheets. If that’s the core you’ve been missing, join the waitlist now.

Choose from Platinum, Gold, and Silver packages
Reach with 25–30% open rates and 0.5–1% CTR
Craft your own custom ad—from banners to tailored copy
Perfect for Crypto Exchanges, SaaS Tools, DeFi, and AI Products